[WIP] Girder python client

@scottwittenburg - Starting a PR here to collect various improvements/fixes to GirderClient.py.  We don't have to merge this yet, if we want to make further changes first.  I'm trying to isolate each change to a single commit so we can discuss the changes more easily.

<!---
@huboard:{"order":202.5,"milestone_order":306,"custom_state":""}
-->