Update readme for easily installing runners

Related to https://gitlab.com/gitlab-org/omnibus-gitlab-runner/merge_requests/3

See merge request !15

#8 install doc

gitlab-ci-runner seems to be less document project, so here are some updates I've made while installing fresh runner today on Centos 6.5.

See merge request !13

Added few CentOS related changes.
Optimized some commands, which won't require changing current working directory.

mac os fix Error: Cowardly refusing to

Typo in RUNNER_PID variable name

Fix upstart instructions in README.md

This should be the same as in `lib/support/init.d/gitlab_ci_runner`.

Send description and tag_list during registration

If specified RUNNER_DESCRIPTION or RUNNER_TAG_LIST can be send during registration. By default hostname is used for description.

See merge request !12

Default file for gitlab ci runner

* Added default file for init script to allow configuration for the
runner
* Updated install area inside ReadMe.md
* Adapted the indention of the switch-case to match the rest of the
script

**EDIT:**

* Fixed description of line 17
* Edited defaultfile variables to match init.d-script variables

Send hostname to Coordinator

Runner sends it's hostname during registration which allows for easier identification.

See merge request !10

Init.d clean

See merge request !15

Update ruby

The old version of ruby can be installed on the recent ubunntu releases because of incompatibility with readline.

http://stackoverflow.com/questions/23488790/compile-ruby-2-0-errors-without-rvm-or-rbenv-readline-c188626-error-func

See merge request !16

Exposed project directory and project id to the build environment

Exposing the project directory and ID will help with automating Docker integration, for example, mapping a directory in a Docker to the build directory, etc.

See merge request !5

Repo url in build script

See merge request !7

Upstart conf's should exist in /etc/init instead of /etc/init.d

Add `-C` option to specify the working directory

The concrete goal of adding this switch is to be able to start a runner as:

```
/opt/gitlab-runner/bin/runner -C /home/gitlab-runner
```

This way we can keep the runner's executable code under `/opt/gitlab-runner`, owned by root, and have `/home/gitlab-runner` (or whatever we come up with) as a writable, working area for the gitlab-ci-runner user.

This new switch does not change the default behavior of the runner.

See merge request !14

bin/setup needs bundler too

By adding `require 'bundler/setup'` we remove the need for `bundle exec`.
This is already done in the other scripts in bin/; this change adds the
missing require to bin/setup.

See merge request !13

By adding `require 'bundler/setup'` we remove the need for `bundle
exec`. This is already done in the other scripts in bin/; this change
adds the missing require to bin/setup.

Add HOME to the Upstart config example

This seems to be a good idea.

See merge request !12

Add Upstart config

Upstart takes care of respawning and logging. This only works for
distributions that use Upstart, such as Ubuntu 12.04, Ubuntu 14.04
and Centos 6.

See merge request !11

Dockerfile update