Check if argument is a coroutine function before checking argument.func

[EvanKrall]

I kept running into issues with asynctest.CoroutineMock and a_sync.block():

>>> import asynctest
>>> import a_sync
>>> import asyncio
>>> a_sync.block(asynctest.CoroutineMock(return_value=5))
<generator object CoroutineMock._mock_call.<locals>.proxy at 0x7f4113e57c50>
>>> asyncio.get_event_loop().run_until_complete(asynctest.CoroutineMock(return_value=5)())
5

This is because CoroutineMock returns a MagicMock for any undefined attribute access:

>>> foo = asynctest.CoroutineMock()
>>> foo.func
<MagicMock name='mock.func' id='139917444876440'>
>>> foo.bar
<MagicMock name='mock.bar' id='139917444622824'>
>>>

Since to_blocking currently checks async_func.func first (which is not a coroutine function), it treats the CoroutineMock like a sync function, even though:

>>> asyncio.iscoroutinefunction(foo)
True

[EvanKrall]

I added tests and a tox configuration for running them (just install and run tox), but the important bit of this change is in a_sync.py. Let me know if you'd rather I take out the tests, or if you prefer a different test setup, etc.