test(ticker,trader,balance): tests created; mocha replaces tap

* added test coverage for Bitstamp plugin,
* tap replaced with mocha,
* `mockConfig.json` file with valid (and confirmed) Bitstamp credentials and $5 available for trading on account required to successfully run **all** tests.

I prefer assert style assertions here: http://chaijs.com/api/assert/

Of course, this doesn't really matter except that we should keep this consistent across all of our tests. So let's spend a few minutes deciding once and for all which style we should use. Any strong feelings against assert style?