Added BSD License

I really like this project and the sensible way it bases the API on existing APIs, but to be able to use it for anything it really needs a clear license. (In my opinion the lack of a clear license has probably scared a lot of people off...) 

Based on [this comment](http://code.almeros.com/android-multitouch-gesture-detectors#comment-15094) I have added a BSD license file crediting you. If you accept this pull request, I would be happy to start using your library and also start working on a two-finger shove gesture (like when you tilt the world in Google Earth).