Provider Verification is broken

[mefellows]

I noticed that the current verify function expects an http response. This is unusual, in most cases we will be calling service collaborators which in turn make the HTTP response calls on our behalf and return back an abstracted object (e.g. in a user service it might return a User object). This means we can only test trivial examples.

See https://github.com/pact-foundation/pact-js/blob/fix/verify/src/pact.js#L103-L116 for an example fix, part of a general API cleanup I'm undertaking with consultation with other community members.