Adding methadone

DO NOT MERGE

This is a PR as discussed in stand-up so you guys can see where I'm going with this. At the very least I need to add a test and clarify an error message.

@mikepea - or any of you - there is a question here 743d83a about usage. Is there a way in methadone similar to `bundle exec thor list` that will show the options of resources to walk?

The next major step will then be removing Thor, after discussion with IDA. I've already spoken to Jeremy and, as I suspected, they use it manually so a change from `bundle exec thor vcloud_walk:vdcs` to `bundle exec vcloud-walker vdcs` is a welcome change because less typing. Will also check with Max/Nick.