Add a juju subcommand for backups.

This will fit on top of the client API that Michael is writing.
So to test that you've correctly Register'd the command, I've generally added a test that "juju backup -h" prints out the expected backupDoc.
If you look it "main_test.go" there are quite a few tests like ""sync-tools", "--help"".

The general idea is that if I can comment out that line, and a test doesn't fail, then we're probably missing something. :)