Adding #match_fedora_datastream rspec matcher

Given that ActiveFedora is an adaptor for CRUD operations on Fedora, I
want to provide Rspec matchers that can greatly assist in validating
that data written to Fedora is written how you are expecting it to be
written.

Adding have_predicate matcher

Adding additional specs for have_predicate

Adding #has_many_associated_active_fedora_objects

Adding rspec matchers for belongs_to

Updating bits of documentation for rspec helper

Finalizing method for including rspec_matcher