Added QueuedConsumer implementation

[tvoinarovskyi]

With Python3.5 support for async with and async for.

NOTE: There are no tests for it. Can't wrap my head around how to implement a separate test for python35, any suggestions?

[TarasLevelUp]

I think this is a little too much to even view at a time. I will split up the Python3.5 and only create a PR for Python3.4 (with tests) later.

[benjamin-hodgson]

I wonder if it might make sense to make this class inherit asyncio.Queue, rather than just wrapping it. Then you'd get all of Queue's methods 'for free' (in particular I think get_nowait might be useful). What do you think?

[benjamin-hodgson]

Regarding testing 3.5-specific functionality - you should be able to just make a test which runs only in 3.5.

if PY35:
    class WhenTestingPython35SpecificFunctionality:
        ...