Add internal/image proto.

Change-Id: I91c78ba0193a2bd91666899964ba6270abc16b41
Add image package.

Change-Id: Ic977cf15567faafc1fa31eb455d261293a223301
Update README to reflect that appengine/image has been ported.

Change-Id: I8d058b2f64696959896534b2e01ba575684e43a9