Move HTTPServer's valid_ip function to netutil.
Add a c-ares-based resolver implementation.
Add a resolver test script for external queries that cannot be unittested.

Note an issue with CaresResolver (it cannot resolve www.dropbox.com when
family==AF_UNSPEC)
Simplify the Resolver interface.

Most callers do not need the full generality of getaddrinfo, and
most alternative resolver implementations cannot provide it.
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.
Make DummyFuture class private.

It's now only exposed and used when concurrent.futures is not available.
Test the IOLoop.current method and add it to TwistedIOLoop.
Add IOLoop.run_sync convenience method.
All functions that take an IOLoop default to current() instead of instance().

This means among other things that it's no longer necessary to pass
IOLoops explicitly in tests.
Release note updates