Proper implementation of configurable relationship names

As promised.

A few notes:
I did write tests, and *they pass*!

I wrote some lying comments initially, and then made them honest (Oops).

These configurable relationship names are ThumbsUp/App-wide, and so cannot easily be used to work with different relationship names from different models.  I don't have this use case, but realized it as I was writing the tests.

The code could easily be enhanced to make `acts_as_voteable` and `acts_as_voter` take an optional parameter to have a custom relationship name each time the relationship is added to a model.

This is a step in that direction, and will make that eventuality much easier to realize.