Association key names

[parrish]

This is motivated by non-standard associations, particularly when the association name doesn't match up with the name of the model. For instance:

class Artist < ActiveRecord::Base
  has_many :records, class_name: 'Album'
end

will check RestPack::Serializer.class_map for 'records' and expect the route to be /records/

I'm not 100% that I'm addressing this in the best way, so I'm really open to feedback on this PR.

Thanks!