Support an :as option on track_history

[johnnyshields]

track_history should support an :as option which will control the alias of the trackable field. This is useful if you move/rename classes, you don't have to migrate your database data.

For example:

module MyModule
  class MyClass
     include Mongoid::History::Trackable

     track_history as: 'my_class'
  end
end

I will implement this in the future (unless someone else wants to)