Adding ability to use getters and setters as opposed to public methods.

Great work on this library. I was always a fan of the syntax in https://github.com/pardom/ActiveAndroid, but the performance hit against reflection on annotations seemed like a downside so I was really happy to see Ollie pop up.

One of the things I didn't particularly like when integrating with my existing model schema was that all the fields had to be public, so I added a new annotation called `@GetSet` that tells the generated classes to look for getter and setter methods instead of accessing the fields directly.