Make the test suite prettier, add coverage

[parente]

I'm not spending many cycles on keeping the test suite beautiful at the moment. Tracking warts here.

• split into multiple modules, not one giant test_gatewayapp module; tests for the jupyter-websocket mode vs the notebook-http mode are a first order candidate for a split
• try to fix the hangs on travis (not fixed, but rather opened upstream Travis stalls after tests succeed #94)
• some test cases are not checking all endpoints when they should be; fix these using loops or by making explicit calls to all endpoints.
• enable code coverage report

[parente]

We've been slowly adding additional endpoint checks as we go. I don't think we need to expend a ton of effort on making sure every single option / endpoint is tested at the moment.

I'll enable test coverage stats next.

[parente]

Coverage stats look fine. Closing.