Add rspec matchers

[todtb]

This may or may not be useful to the larger community, but here they are if interested. Love to hear any feedback either way.

These matchers are a suplement to the existing schema specs. The schema (by necessity) cannot assert against specific adapter dependent options (PK, FK, etc), however when using with AR (or otherwise passing such options) this helps assert those are maintained.

# example:
expect(subject).to belong_to_resource(:userr).with_options(foreign_key: :user_id, resource: User)
       expected that PostResource would belong to user
       expected that foreign_key would be user_id, was user_idd
       expected that resource would be UserResource, was UsrRsource