OAuth: Support refresh tokens, etc

OAuth: Support refresh tokens, etc

Initially I was going to write a failing test for refresh tokens. Thus
this fix includes an orphaned 'expect_failure' method in test utils.

I ended up writing support for OAuth refresh tokens, as well as a lot of
cleanup (hopefully) in the OAuth plugin code.

**Rebase**: While waiting for this stuff to be merged, the testing
framework changed, it comes with batteries included regarding fails.
Removed legacy nosetest helper.

Also added a lot of backref=backref([...], cascade='all, delete-orphan')