feat(runner): add beta support for using mocha as your test framework

This change adds limited support for using mocha as the test framework instead
of jasmine. Make the switch by using `--framework=mocha` on the command line
or adding `framework: 'mocha'` to the config. Tests will be run using the
BDD interface. The interface is adapted so that tests run asynchronously
without needing to call `done()`.

Note that there is currently no support for an assertion framework, so you will
need to require whichever assertion framework you prefer. This means there
is no adapter to make the assertions unwrap promises, so you will need to
resolve promises yourself and run the assertions afterwards.