Destroy machine when last principal unit removed.

This change causes a machine to be automatically removed when the last principal unit running on that machine is removed (remove-unit). If the machine is a parent to any containers, it is left alone.

Background:
https://bugs.launchpad.net/juju-core/+bug/1206532
https://bugs.launchpad.net/juju-core/+bug/1183309/comments/8

This could use more test coverage, but I'd like to get a general feel for how it's looking so far before I go much further with it. This is my first time mucking around with state transactions. Please let me know if I've got it right!

I haven't added an option to keep empty machines/containers around after removing the last unit. I'd like to see if we can avoid it. If we do need the option, I think it should be opt in. Let me know.
I don't think this is quite right -- if we *shouldn't* destroy the machine, don't we want to $or together the reasons-not-to-destroy? And if we *should* destroy, we'd want to $and together the reasons-its-ok-to-destroy. Right?

I think we'd have an easier time of it if we expressed it as cleanupMachine*Ops* (or maybe Op) and built them all in the one method, and just used that alone here.