Accumulated release note updates.
Add @gen.coroutine and gen.Return for future-based (and py33-style) coroutines.
Convert auth methods from return_future plus gen.engine to gen.coroutine.
Test twisted and cares resolvers in netutil_test.
Convert resolvers to @gen.coroutine.

This adds a requirement that there be a "current" IOLoop, which introduced
a few ripples.
Fix resolver test script.