Move AmazonS3 command line utility to jgit-pgm

This removes one of the few remaining dependencies on AWTAuthenticator
from the core library.  For the most part the interface is identical to
the prior main method.  The jgit-pgm Main class already sets up the
HTTP proxy and authenticator for us, so we don't need to do that in
our new run method.

Change-Id: Ia2cce34d34c97b88214a8bd8f2cc542845e19032
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>