Create Wiki migration task.

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.