Replace DeferredUtils with Observable return value handler

[rstoyanchev]

It should be easy to create a custom return value handler to allow controllers to return an Observable. In other words plugging the code that's in DeferredUtils into Spring MVC. Something like this here.

[rstoyanchev]

Here is a pull request #4.

[cer]

That's awesome. Your suggestion was very much appreciated. The PR is even better!