Move add and remove machine to subcommands.

Add a subcommand for "machine", and move "add-machine" and "remove-machine" in to it.

Automatically deprecated aliases are added for the existing "add-machine" and three different variants of "remove-machine" to point to the new subcommands under "machine".

The tests for the machine subcommand completely mock out the API.

There is one test for each command in cmd/juju/machine_test.go that shows things are hooked up.
I did check that the api server has exhaustive checks around both add and remove machine already.