Test coverage & refactor

+ Tested endpoints and API-common methods (parseBody, GET & POST) with
mocha.
+ Applied the single responsibility principle the best I could, now:
  + buffer-api knows which endpoint call for each of its public methods.
  + Each endpoint knows which url must hit, and if it is a POST or a GET
  request.
  + api.coffee handles the response parsing and callback handling, and
  knows how to parameterize GET and POST requests.
+ Added sample.coffee file, which shows a little example of API usage.