text: Title: Migrate from "yield from" to await (TypeError: object Engine can't be used in 'await' expression)
  Body:  Hi, i replaced in my code "yield from"  to "await", and received Traceback:
  "TypeError: object Engine can't be used in 'await' expression"
  
  ``` python
  async def db_psql_middleware(app, handler):
      async def middleware(request):
          db = app.get('db_psql')
          if not db:
              app['db_psql'] = db = await create_engine(app['psql_dsn'], minsize=1, maxsize=5)
          request.app['db_psql'] = db
          return (await handler(request))
      return middleware
  
  
  async def psql_select(request):
      with (await request.app['db_psql']) as conn:
          result = await conn.execute(models.select())
  ```
  
  Traceback
  
  ``` python
  [2015-09-17 14:50:29 +0300] [26045] [ERROR] Error handling request
  Traceback (most recent call last):
    File "/Users/vvv/src/backend-tools/python/asyncio/venv35/lib/python3.5/site-packages/aiohttp/server.py", line 272, in start
      yield from self.handle_request(message, payload)
    File "/Users/vvv/src/backend-tools/python/asyncio/venv35/lib/python3.5/site-packages/aiohttp/web.py", line 85, in handle_request
      resp = yield from handler(request)
    File "/Users/vvv/src/backend-tools/python/asyncio/app.py", line 39, in middleware
      return (await handler(request))
    File "/Users/vvv/src/backend-tools/python/asyncio/app.py", line 46, in psql_select
      with (await request.app['db_psql']) as conn:
  TypeError: object Engine can't be used in 'await' expression
  ```
  