Add async helper

[Steven-Chan]

Since the whole thing is originated from a js framework, it's reasonable to assume it works best on main UI thread. But a lot of cases, developers would like to do some async actions, e.g. network request, database query.

I think it would be very nice to have a helper function or class or something to help developers to write async code and, immediately right after that, dispatch action in main thread. Otherwise, you may easily come up with some callback hell.