Use "from unittest import mock". It should simplify my work to merge new tests
in Trollius, because Trollius uses "mock" backport for Python 2.

Thanks to Vajrasky Kok for discovering some of them.

DeprecationWarning warnings. create_unix_server() closes the socket on any
error, not only on OSError.

Mention if the error was caused by a read or a write, and be more specific on
the object (ex: "pipe transport" instead of "transport").

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

asyncio: Add close() back to Unix selector event loop, to remove all signal handlers. Should fix buildbot issues.

This should have no effect, it's a "shouldn't happe" case.
Also tidied up some comments.

Rename Transport.pause/resume to pause_reading/pause_writing. Also relax timeout in test_call_later().