tinyio v0.2.0

A couple of breaking changes to tidy up our interface:

• Removed tinyio.add_done_callback. This was literally just out = yield coro; success_callback(out) so it's pretty trivial for a user to write as a wrapper coroutine themselves (and in the same number of function definitions, as they previously had to write the callback anyway).
• Switched from iterator = tinyio.AsCompleted(...) to iterator = yield tinyio.as_completed(...). In particular these tasks are scheduled in the background at the moment of this new yield, rather than on the first get. This should make multi-consumer cases a bit more natural.

I think I'm pretty happy with what the API is now settling down to look like.

Full Changelog: v0.1.4...v0.2.0