Add json_content, get/put/post/head/delete

As an attempt to use this, I am trying to replace our use of `mock_treq` in `test_history`, since that is a super small use case.  Our patched treq needs to return deferreds :|

Probably not done, but here's what I have so far.  Have to run for now.
This and below change might break #654 tests but this is better :)