Bug 1078520 - python client

Implement a python client.  Note to reviewers, there were a couple fairly substantial changes to the way the module in the latest commits.  It's probably best to only really look at the most recent code.
This is a left over from an earlier version of the `authorizedScopes` feature... That earlier version had a very bad name too... Absolutely delete this. I'll remove it from taskcluster-client for node.