[[_]] float clearing tag experiment

Fix symlink base path, don't allow in bare repos

Basic support for symbolic links

Update references to new project location

Fix tests that depended on old github location

* Change 'github/gollum' references to 'gollum/gollum'

* Two tests depended on the following file existing:
  github/gollum/master/test/file_view/1_file.txt

Fix #645

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

  * Empty commits via editor now have the message: "[no message]"
  * Reverts now have the slightly more useful message: "Revert commit #1234567"

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.

Update working directory when creating new page in subdirectory

Allow registering custom markup engines

0.4.1 and 0.4.2 break gollum's tests

org-ruby 0.8.0+ break the tests

This required adding a file to the example repo, so some of the tests'
expected hash values changed accordingly.

https://github.com/github/gollum/issues/636

Fix Create New Page and Rename Page dialogs redirect behaviour

Specify the sidebar side

Fix redirects to index_pages for subdirs