Create RequestPattern without RequestStub

This refactor is to make possible an item I sent out to the mailing list a while ago:
> Pacto.contract_for should work even WebMock is not used (Fog stubs, VCR, possibly Pacto Server)

This adds the internal capability.  The public interface will be added after merging this into #52, or creating a new PR to provide a stub equivalent to Pacto.generate! and Pacto.validate! (probably not Pacto.stub! because it conflicts with rspec-mocks).

Note: method name stub_request! is a bit of a misnomer now since it may or may not setup a stub.  I'll save that refactor for the public interface PR.