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