Minor fixes

Hi Felipe, I hope you can accept my changes.

I just added more userless request functionality to nearby tips, trending venues, and venues detail. I also removed some unused imports and fixed a typo in the Venue model.

I think this library could be improved in the following ways:
 - add a "global" intent to get nearby venues in `VenuesCriteria.VenuesCriteriaIntent` [see here](https://developer.foursquare.com/docs/venues/search) for the intent.
 - fix the README
 - fix a few more typos
 - add some strings to `strings.xml`
 - maybe include the possibility to initialize this library with the client id and secret
 - maybe include a way to choose if I want a ProgressDialog with my request
 - add some more methods for all the [API Endpoints](https://developer.foursquare.com/docs/) in the Foursquare API

I would be happy to help and to explain my changes.
Thanks