Added acts_as_rateable functionality and unit tests.

I have added some optional functionality for models that act as rateable, so that apps can determine user behavior.

To this end, if you add:

    acts_as_rateable

to a method, you will be able to use the methods

    @movie.person_ratings
    @movie.rating_of(person)
    @movie.has_been_rated_by?(person)

A couple of other utility methods were added as a result:

    Coletivo::Models::PersonRating.find_all_by_person(person)
    Coletivo::Models::PersonRating.find_by_person(person)