Merge branch 'gitlab-gem' into 'master'

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