Document allowable default_version settings for git sources.

accomidate hardlinks properly

now with tests

ensure the destination folder is created for travis

create destination for hard link file_syncer spec

more test fixing

Forcibly remove a non-empty project dir before cloning

If multiple projects are using the same maching for building, they
might have two different software definitions that share the same name
but a different source. For example, the old omnibus-chef-server
repository uses a git-based source for bookhself. However, the new
chef-server repository uses a path-based source. If the new
chef-server project builds before the omnibus-chef-server project, the
build will fail with an error message from git complaining about a
non-empty directory.

NB: This clearly doesn't solve all the problems with having two
projects building on the same machine with different fetchers for a
given software. For example the two software definitions could both
use git but via different git upstreams.

Add Seth Vargo as a maintainer

We don’t have dedicated lieutenants over individual subsystems yet.

Allow the use of SHA checksums in source definitions

[skip ci][docs] Fix typo in cache doc

Probably the most important commit ever :)

Add MAINTAINERS file

Correctly determine git repository when running in a subdirectory

When running in a subdirectory of a git repository, .git won't exist.
Instead of looking for ./.git specifically, attempt to get the
revision and rescue if we can't.

Update builder.rb

Use double-quotes since we are referencing a variable

ignore ctag files

ignore ctag files for gems
ignore hidden ctag files
names for the file aren't connonical
because: grammar

`generate changelog` can now generate changelog between two refs

This commit add support for the following:

- optional arguments for the starting and ending revisions to base the
  changelog off of.

- reading the start and end revisions from the appropriate version
  manifests

- reading the new version from the version manifest

Override equality operator for ManifestEntry

Prior to this, we simply tested basic object identity, which would
result in many "updated" dependencies like this:

```
* libossp-uuid (1.6.2 -> 1.6.2)
* postgresql (9.3.5 -> 9.3.5)
* popt (1.16 -> 1.16)
* cpanminus (1.7004 -> 1.7004)
* rsync (3.0.9 -> 3.0.9)
* libyaml (0.1.6 -> 0.1.6)
```

Now, equality of `ManifestEntry` objects is in terms of their
data. This will remove the spurious "updates" above

With tests!

Update chef-sugar to 3.0 which adds ppc64le support.

Cache Builder#shasum before Builder#build to ensure consistent result

dpkg uses arch name ppc64el (instead of ppc64le) for little endian.

The Builder#shasum is based, in-part, on the build_commands in the
instance of Builder. This is problematic because the `block` DSL
method allows users to use other Builder DSL methods inside of it, all
of which mutate the build_command array when #build is called. Thus, a
given builder will return a different shasum before and after build is
called.

Caching shashum at the beginning of build fixes this problem, but has
a major drawback. Namely, two builders with the same shashum might
execute different commands.

This problem is partially mitigated by the fact that the builder
shashum is just a part of the software shasum, which will typically
include the actual text of the software definition as part of its
digest.

This will not catch software definitions where the build steps can
produce different based on the system state, such as this:

  block do
    Dir.glob("#{install_dir}/embedded/apath/bin/*").sort.each do |f|
      link bin, "#{install_dir}/embedded/bin/#{File.basename(f)}"
    end
  end

However, that will always be a problem since we allow pure Ruby in our
build steps.

Fixes #450

Respect append_timestamp config when constructing a version number

Previously, Config.append_timestamp was only respected by the
Omnibus::BuildVersion.semver function. This option is now respected by
the dsl-based version builder as well, ensuring that even when the
user specifies an explicit version with a string, a timestamp is
appended.

This is a behavior change for some users. Users who do not want a
timstamp can alter their configuration.

We've added backwards compatibility for those using the 'semver' style
versions to ensure that we do not append a timestamp to a version
already containing a timestamp.

Add build_version and build_git_revision to manifest

This commit adds the build_version and the build_git_revision to the
version-manifest.  The goal of these additions is to make it possible
to tag and generate a changelog for a given build using two
version-manifest.json files and the omnibus project's source code
repository.

Add `omnibus generate changelog` command