Return result instead of Future from os funcs wrapper

[gyermolenko]

Since support for python3.4 was dropped from the lib I've tried to change from yield from syntax to awaits.

Immediately got into AttributeError: 'Future' object has no attribute st_size in test_os.py::test_stat. Change is small but in return type, so I decided to show it separately.

And another one: here __iter__ has @asyncio.coroutine. I don't think it should (and tests pass without it too :) )

[gyermolenko]

hi @Tinche , just a ping

[gyermolenko]

hi @Tinche .
Just a kind reminder.. I am very interested in your thoughts about this PR.
(about yield from -> await transition; @asyncio.coroutine above __iter__ etc).

[Tinche]

Thanks!