Issue #275: Fix asyncio for Python 3.4.2

Don't use asyncio submodule (asyncio.tasks), iscoroutine() is directly
exposed in the asyncio namespace. In Python 3.4.2, iscoroutine() was
moved from asyncio.tasks to asyncio.coroutines.
Merge pull request #278 from haypo/master

Issue #275: Fix asyncio for Python 3.4.2