Support GeocodeFarm geocoder

This pull request is for adding a geocoder for GeocodeFarm.com:

https://www.geocodefarm.com/

Comes complete with:

* New geocoder that can `geocode` and `reverse`
* Full test suite
* Updated documentation (not really tested)

As far as I can tell, it's good to go.  Let me know if there's anything else that should be done.

Oh, and I also should note that I didn't add any `..versionadded` stuff to the docstrings, but I'm happy to do so if you know what the version would be.