Converted Camera's local rotations from Euler method to Quaternions

Prior to this pull request, I added support for rotating the `Camera` when `mLookAt` is set but realized the movements were jittery when the rotations were set in look at mode (which never happens in the RajawaliExamples). Also, I noticed there were already necessary methods for applying rotations in using the `Quaternion` class. 

So I basically cleaned up my previous code and converted local Euler rotations to Quaternion based rotations to avoid [gimbal lock](http://www.youtube.com/watch?v=zc8b2Jo7mno).

Instead of adding `pitch()`, `yaw()`, and `roll()` methods, I changed the code to reuse the `setRotX/Y/Z()` methods.

Example
-------------
```java
public void onDrawFrame(GL10 glUnused) {
   // Set the camera to follow around the object
   mCamera.setLookAt(mObject.getPosition());

   // Roll the camera based on its local axis
   mCamera.setRotZ(mCamera.getRotZ() + 0.1f);

   super.onDrawFrame(glUnused);
}
```