Added Accelerometer Input Processor for android

Hello,
InputProcessor abstract class does not provide methods that get triggered when the accelerometer value changes so I have figured out it to be difficult to work with libgdx when I need to use accelerometer. So I have added feature like InputProcessor abstract class for Accelerometer. I have added a new abstract class that extends InputProcessor so that it does not cause conflict with existing demos and games. 

I have created a working demo using this: https://github.com/sadaf2605/Libgdx_AccelerometerInputProcessor_Test_Android_App

This is the first time I am trying to contribute on a big cool opensource project what I always wanted to do. So at least please let me know your feedback!