Improved Photo Rotation API

[Originally Requested [here](https://github.com/chrisbanes/PhotoView/pull/151#issuecomment-35374038)]
During development, I realized that the `setPhotoRotation` method existed (did not find at first because of the name). Once I used it I realized that it would only set the rotation from zero degrees - not from the current rotation matrix. 

Over a few revisions, I was able to improve the API by adding the methods `setRotationTo` and `setRotationBy` that rotate from zero or from the current rotation, respectively. Additionally, these do not conflict with any `View` methods, so these integrate better for someone looking for a rotation method to use.