Add CameraKit functions

[austinkettner]

.getBitmap();
CameraKitView.stop(); = onPause()
CameraKitView.start(); = onResume()

As per #318 comments

Official Maintainer Ticket.

[emersoncloud]

From @polppol on the PR:

replace stop() with onPause() work fine
but when I call onResume() it just crashed

java.lang.NullPointerException: Attempt to invoke virtual method 'int com.camerakit.CameraKitView$Size.getWidth()' on a null object reference
at com.camerakit.CameraKitView$Camera2$1$7.run(CameraKitView.java:1844)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.os.HandlerThread.run(HandlerThread.java:65)

EDIT: Only crashed if captureImage() is called right after onResume()