StreamWriter: close() now clears the reference to the transport

StreamWriter now raises an exception if it is closed: write(), writelines(),
write_eof(), can_write_eof(), get_extra_info(), drain().

StreamWriter now raises an exception if it is closed: write(), writelines(),
write_eof(), can_write_eof(), get_extra_info(), drain().

- Add a new _wakeup_waiter() method
- Replace _create_waiter() method with a _wait_for_data() coroutine function
- Use the value None instead of True or False to wake up the waiter

loop parameter is not set

Add unit tests to ensure that constructor of StreamReader and
StreamReaderProtocol classes get the event loop.

Replace also _make_drain_waiter() function with a classic _drain_helper()
coroutine.

- Log stdin, stdout and stderr transports and protocols
- Log process identifier (pid)
- Log connection of pipes
- Log process exit
- Log Process.communicate() tasks: feed stdin, read stdout and stderr
- Add __repr__() method to many classes related to subprocesses

The create_task() method can be overriden in custom event loop to implement
their own task class. For example, greenio and Pulsar projects use their own
task class.

The create_task() method is now preferred over creating directly task using the
Task class.

Oops, I wanted to send this patch for review before

In the greenio project, Task._step() should not create Task objects but
GreenTask to control how tasks are executed. Luca Sbardella already asked this
feature for its Pulsar project to support coroutines using yield instead of
yield-from.