added --use-async option: generate async handlers (tornado)

[ptytb]

No description provided.

[Weilor]

The modification could not be compatible with python 2.x, though python 2.x should not be supported anymore.
Maybe we can publish a new major version to accept this pr? @rejown @zrq495 @foodszhang

[ptytb]

For python prior to 3.5 (without the async keyword) it is possible to use @gen.coroutine decorator on handler methods and use yield instead of await to make them asynchronous.

So there could've been two mutually exclusive options like --use-async and --use-gen-coroutine.