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.

`should`/`expect` sound more semantic/like a sentence when reading, and make it easier for any reader to understand. Also, I'm already writing more tests for `lamassu-server` using `should`. I think it'll be better to stick with it.