Also moved the apiKey to be a class property that can optionally be specified by the caller. If not specified, the Android app's key is used.
Finally, made 'headers' optional for ShiftRequest::_request(). If not specified, the full set of headers, including API key, is used.
Basic structure and proof of concept for talking with Shift via Node. Will eventually expose the remaining (known) APIs, but this was to get myself familiar with promises, http, streams, etc.
