RtlsdrAio improvements

[nocarryr]

These are mostly fixes for bugs that were encountered when using the subprocessing and multi-threading
methods that work behind the scenes in py-test (while trying to tackle something mostly unrelated to asyncio).

Since the read_bytes_async is called in an Executor, it seems appropriate to ensure that all of the async functions can be handled properly when the stop() method is called.

• Place samples received from the _callback into the queue using a coroutine.
• Catch exception raised if the queue is full (asyncio.QueueFull) and log the error.
• Use coroutines to start and stop AsyncCallbackIter to ensure that all remaining coroutines can be "awaited"
• Update the test case to await the stop() method, add test for read_bytes_async
• Update the example in the README to include the await expression for stop()

This PR has a lot of ugly commits from fighting with travis-ci and its outdated default python3.5 package (3.5.0.. current stable is 3.5.2). As such, I think it should be a squash merge.

[coveralls]

Coverage decreased (-0.1%) to 85.958% when pulling d4bc47f on nocarryr:aio-changes into 3479723 on roger-:master.

[coveralls]

Coverage increased (+0.2%) to 86.325% when pulling 94207fb on nocarryr:aio-changes into 3479723 on roger-:master.