Adapter for gollum to use Grit at the backend.

Make sure the git adapter specs pass by running rake, but also make sure that the gollum-lib specs pass when using your branch of this adapter as a backend:

• Clone the latest version of gollum-lib.
• Change gollum-lib's Gemfile so as to use your local version of this adapter, i.e.:

-gem 'gollum-grit_adapter'
+gem 'gollum-grit_adapter', :path => "/path/to/your/gollum-lib_grit_adapter"

• Run the gollum-lib specs and see if they pass.