Switched to a general lib for timestamped serializers

Since all our models will implement this pattern, let's
codify it
Added webhook serializer

Webhook dynamically calculates its signature, however
that calculation is irrelevant for ensuring it is
serialized properly. So signature is stubbed out.

This required adding a stubbing/mocking framework.  Although
this wasn't officially discussed, I've run into this issue on
several occasion: needing to stub or mock where minitest/mock
was insufficient.

I chose flexmock because it's very small, lightweight,
has a namespace that doesn't conflict with any other library,
and served the purpose.

Should the community choose otherwise, it would be easy to replace
later with something else.
Merge branch 'serialize_all_the_models' of git://github.com/drewblas/web into drewblas-serialize_all_the_models

Conflicts:
	Gemfile.lock