V4: Support HTTP and Message interactions in the same test

[uglyog]

Master tracking issue for V4 spec https://github.com/pact-foundation/pact-specification/issues/79

This issue is to track all problems that arise from combining both HTTP and message interactions in any required form.

Current Status

V4 models and Pact files have been implemented. Still required to replace all uses of concrete Pact classes with interfaces to resolve class cast exceptions.

[solarmosaic-kflorence]

Highest priority for me is just having the ability to merge message pacts and request/response pacts into the same pact file, and having the framework support reading one or the other as necessary. I only need this support in the base Consumer and Provider JARs.