Implemented support for polymorphic 'belongs_to' association

Brainstem::Presenter#load_associations! now:
  - supports polymorphic 'belongs_to' associations
  - represents the polymorphic association attribute name by adding '_ref' postfix 
  - polymorphic 'belongs_to' association is represented as a hash which includes:
    -- id: The id of the polymorphic object
    -- key: The table name for the class of the polymorphic object