Adds support for projects using multiple ORMs

As described in #81, the presence of Mongoid in the project loaded `Rails3JQueryAutocomplete::Orm::Mongoid`, and not `Rails3JQueryAutocomplete::Orm::ActiveRecord`. Instead I:

* loaded all ORM specific files, 
* name spaced methods with duplicate names (`get_autocomplete_order` and `get_autocomplete_items`), and 
* figure which method to call based on the Model's superclass and included modules.

Finally I changed the tests to match the new method names.