Generate acceptance tests

Adds API's concept of acceptance/integration tests to Pliny. The main goal of these versus the unit tests on endpoints is as follows:

1. Exercise entire HTTP stack including middleware (i.e. operates on `Routes` instead of endpoint).
2. Test for response regressions. Eventually this piece will be JSON-schema backed.
3. Avoid mocking/stubbing so that interactions between all objects in the stack can be executed without interference.

Fixes #41.