Initial libuv support

Hmm, the 32-bit builds all failed with 'The system cannot find the drive specified.' five times. Maybe it didn't like our comments?

This requires some changes to the way we handle errors, specifically
for signals in the monkey-patched test_selectors.py and
test_socket.py.

It's not absolutely clear that this is the right thing to do in all
cases.

Fix #1044 by always closing opened sockets before raising

Account for some more warnings that Python 3.6 can generate: ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__. See https://travis-ci.org/gevent/gevent/jobs/310040846

Also enable ResoureWarnings by default in the test suite and fix a
bunch that showed up.

Make universal newlines with FileObjectPosix return str on Python 2 i…

Upgrade c-ares to 1.13.0. Fixes #990