Prettify api client code

Remove callbacks, needs more investigation
Minimal behind-the-scenes session token handling
Start using model with SessionToken
Initial bunch of tests!

Do not use objects at api level, pass-through to high-level for
wrapping.
Tests, add api_key to get_user_session
Add folder/get_info
Resolve resources by path and id
Fix uri/action transition
Hack download support
Use command line for credentials

... until I make up my mind on where to store them.

Add stat action