Review Board 4.0 has a better architecture for writing hosting service

client implementations, which takes care of a lot of cross-Python

compatibility issues, simplifies code, and reduces the need for certain

hacks and workarounds and specialized HTTP access wrappers we had

before.
This change rewrites RB Gateway to leverage the new client support. The

ReviewBoardGatewayClient class is responsible now for all API

communication, handling errors and headers in consistent ways. The

ReviewBoardGateway hosting service class simply calls into it and

converts payload results into Review Board objects.
This will improve maintenance as we go forward, and also takes care of

some differences in behavior between Python 3.x releases that we were

previously hitting.

Unit tests pass.
Manually tested authenticating, linking a repository, browsing commits,

posting for review, and viewing diffs. Aside from some bugs within

RB Gateway itself, this all works.