Make rest2html work with docutils 0.13

See merge request !5

Disable raw rendering

See merge request !4

gitlab-markup gem

Renamed the gem and related documentation.
 
Published a pre-release version on rubygems.org: https://rubygems.org/gems/gitlab-markup.
Please name a few other maintainers I should link to in rubygems.org to give access to the gem.

Merge this MR if no other feedback is needed here and I will commit to master a non-pre-release version and update the MR in gitlab-rails: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6659

See merge request !3

Python3 support

This branch is based on GitLab-CI support, and should be merged after !1.

We added support for both `python2` and `python3` in the codebase, but it still has python2 hard-coded.

For now we are patching the interpreter used in omnibus to change it to python3 to keep backward compatibility with users using it by source.

In the near future, we will make this switchable with the help of an initializer.

See merge request !2

GitLab-CI support

What was done:
* Converted `.travis-ci.yml` to the equivalent in `.gitlab-ci.yml` and added additional rubies.
* JRuby support is removed as we don't use and a dependent gem removed the support for it: https://github.com/jgarber/redcloth/issues/31#issuecomment-252954871
* Fixed `README.pod` fixture to use a valid markup
* Fixed `README.pod.html` to include updated expected render.

See merge request !1

This code will keep current behavior for python 2.x.

Conflicts:
	lib/github/commands/rest2html

Added doctest blocks to reStructuredText (with tests)

Many Python projects use Sphinx to create documentation from .rst markup.

Using a doctest:: directive the markup can include blocks of code that is both
tested and rendered in the final documentation. These blocks are not rendered
on github.com though, e.g. in a README.rst.

To remedy this (without adding Sphinx as a dependency) this PR aliases
doctest:: directives to code:: directives,. The language of such blocks is set
to 'python', since doctest only supports Python.

Refs dateutil/dateutil#83

Resolve issues with the `tt` tag in MediaWiki