Add support for Spring 3.2/Servlet 3 Async integration

I'm sorry if there is already an issue for this, I couldn't find one.

Spring 3.2 adds support for Servlet 3.0 async responses, using a Callable or DeferredResult. I see that Spring Security is releasing a new version with integrated support (http://www.springsource.org/node/3766).

It would be handy to be able to return a Callable or DeferredResult from a webflow action as well.