GPUImageVideoCamera: make class methods to determine whether front and or back facing cameras exist

The new iPod Touch doesn't have a back camera, so it's useful to determine which cameras exist before initting the GPUImageVideoCamera.

I have left instance methods of the same name so that the property getters still work, there are just class method alternatives now too.
Merge pull request #1045 from karlvr/nobackcamera

GPUImageVideoCamera: make class methods to determine which cameras exist