Add new functionality

1, add like Instagrm focusing function.
2, add focus animation component.
3, add nitification for Session
4, add notification for capture photo
5, add the delegate to camera
fixed bug for session startRunning and stopRunning
Switch between back and front camera uses SCCameraDevice directly instead of the useFrontCamera property.

I think, to use enumerations for setting will be better?
Just a few points

1、didCapturePhoto need to start animation
fixed bug and add KVO

1、fixed handleRetakeButtonTapped
2、add KVO with FocusMode change
3、fixed KVO with adjustingFocus
Fixed some English.
Added support for SCFlashMode in every sessionPreset