Accumulated release note updates.
Flushing the buffer when calling IOStream.read_until_close()

Ran into this when calling IOStream.read_until_close() after calling
IOStream.read_until("\r\n"), leaving data in the buffer.
Added test_streaming_callback_with_data_in_buffer to iostream_test.py
Python3 compatibility for test_streaming_callback_with_data_in_buffer
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.
Merge pull request #686 from omribahumi/master

Flushing the buffer when calling IOStream.read_until_close()
Fix twisted version check.