This commit adds a new Rake task for migrating all of your existing
Wiki content from your database into new Gollum repositories.
The bulk of the logic happens within the `WikiToGollumMigrator`
class which is decently test covered and located in the lib directory.
The new Rake task can be executed by running:
`bundle exec rake gitlab:wiki:migrate`
It will output a nice log of every project that it migrates along
with success or failure messages.
I have used it on my own installation to migrate my Wikis successfully.
