Update asyncio from the Tulip project

Major changes:

- StreamReader.readexactly() now raises an IncompleteReadError if the
  end of stream is reached before we received enough bytes, instead of
  returning less bytes than requested.

- Unit tests use the main asyncio module instead of submodules like events

- _UnixWritePipeTransport now also supports character devices, as
  _UnixReadPipeTransport. Patch written by Jonathan Slenders.

- Export more symbols: BaseEventLoop, BaseProactorEventLoop,
  BaseSelectorEventLoop, Queue and Queue sublasses, Empty, Full

Lib/asyncio/selector_events.py

@@ -4,6 +4,8 @@ A selector is a "notify-when-ready" multiplexer.  For a subclass which
 also includes support for signal handling, see the unix_events sub-module.
 """
 
+__all__ = ['BaseSelectorEventLoop']
+
 import collections
 import errno
 import socket