Fix #636

https://github.com/github/gollum/issues/636
Add system for registering new page extensions / markup formats
Add test for custom markup engine

This required adding a file to the example repo, so some of the tests'
expected hash values changed accordingly.
Update pygments
Notify me on build failure
Fix tests

org-ruby 0.8.0+ break the tests
Fix tests

0.4.1 and 0.4.2 break gollum's tests
Merge pull request #637 from jamesdabbs/custom-markup

Allow registering custom markup engines
Update working directory when creating new page in subdirectory
Merge pull request #641 from jamieoliver/bug/new-page-subdirectory

Update working directory when creating new page in subdirectory
Update org-ruby. Fixes #640
Update pygments.rb. Fixes #639
Document how to run the tests
Update gollum.gemspec
Update Gemfile
Update gemspec
Update the GFM link in the README

github.github.com/github-flavored-markdown shows a different version of
the page for a split second, and then redirects to
https://help.github.com/articles/github-flavored-markdown so let's just
point there directly.
Merge pull request #643 from mattr-/master

Update the GFM link in the README
Fix empty commits and non-descript revert messages

  * Empty commits via editor now have the message: "[no message]"
  * Reverts now have the slightly more useful message: "Revert commit #1234567"
Merge pull request #644 from dekimsey/empty-commit-612

Fix empty commits and non-descript revert messages (For #612)