Platform gesture

Provide gestures interfaces for both iOS and android.

Gesture recognition is implemented explicitly on individual platform, handled in the core by normalizing the gesture inputs from both the platforms.

This implementation provides naive handling from default gestures.

Some of the main TODOs include: 
a. Handling of continuous zoom
b. Account for zoom levels when panning.
Since the renderer is running on a separate thread from the UI thread, we might need to post input events as a queued runnable to be truly thread-safe (see the "Handling Events" section here: http://developer.android.com/reference/android/opengl/GLSurfaceView.html )