Allow native coroutines to be test functions

[etataurov]

This will allow to write tests using new async def/await syntax.

References:

• Original problem that tornado.gen.coroutine does not work properly with native coroutines seen here https://github.com/tornadoweb/tornado/pull/1550
• Similar pull request for pytest-asyncio https://github.com/pytest-dev/pytest-asyncio/pull/17
• How to exclude python3.5 specific tests https://github.com/pytest-dev/pytest-asyncio/commit/67ab432978bcf0f67293c2fbc8d3fce6d141de04

[etataurov]

Seems I forgot to check older tornado versions, also I need to fix travis.yml to not to run old pytest&tornado under python3.5

[etataurov]

Build failed again. For 2 reasons:

1. I forgot to exclude old pytest for python3.5
2. Newly added test for run_sync=False uncovered that this option doesn't work with tornado=3.2. This needs to be fixed somehow: real fix, drop tornado3 support or mark this feature as tornado4 only

[etataurov]

@eugeniy please take a look

[eugeniy]

Looks good, thanks!