rsync special remote

Fully tested and working, including resuming and encryption. (Though not
resuming when sending *with* encryption; gpg doesn't produce identical
output each time.)

Uses same layout as the directory special remote and the .git/annex/objects/
directory.

Remote/Git.hs

@@ -158,7 +158,7 @@ rsynchelper :: Git.Repo -> Bool -> Key -> FilePath -> Annex (Bool)
 rsynchelper r sending key file = do
 	showProgress -- make way for progress bar
 	p <- rsyncParams r sending key file
-	res <- liftIO $ boolSystem "rsync" p
+	res <- liftIO $ rsync p
 	if res
 		then return res
 		else do