Create a test runner for BridgeDB unittests

We need a test runner which can pass on commands (i.e. doing "python setup.py test" would actually run "trial coverage [args] [tests/test directory]" and pass the arguments and test cases to coverage if need be. See tahoe-lafs's code for an example) and environment variables (i.e. PYTHONPATH).

Ideally, this thing will handle coverage (but that shouldn't be prioritised, as it isn't necessary), and it should handle whatever different unittest frameworks are used so that all pertinent tests can be run with one command.