WIP:  Adding coverage reporting

Attempting to add Istanbul coverage reporting on browser tests run via mocha-phantom. Hrm....

I have a gulpfile that has some tasks in it that:
* instrument source files w/Istanbul (running first through React jsx transformer) and that pipes those files into a temporary directory under /test/;
* run tests via mocha-phantom, passing hook through as an option to run coverage reporting against __coverage__ object once tests are finished 

What I think is missing:
* currently using abspaths to assets in test runner options (for path and hooks) for the purposes of wiring things up -- do I need to run a proxy server to serve test files to localhost address instead of //file:...?
* Not sure how to run tests against _instrumented_ files. There's some kind of connection here that I'm not understanding how to make (I understand that we're running this stuff as middleware in Sprintly and doing the instrumentation on the fly -- is that what needs to happen here? I'm trying to go for a more pared-down setup here, but maybe that's not possible?)