Add API version to requests

per the new requirement for a `&v=YYYYMMDD` appended to all request query strings (see https://groups.google.com/forum/#!topic/foursquare-api/X60gF5Zi0k4)

This PR allows for the version to be set during instantiation of Foursquare::Base, or alternatively fall back to the default of sending today's date in `YYYMMDD` format.