Chainable methods on surface.

Addressing: https://github.com/Famous/famous/issues/110

@AdrianRossouw asked if we could make the on method chainable as well. However, it doesn't make sense to implement `.on` as chainable only in Surface, it should be consistent across the framework. If we agree that we want to make eventHandler's methods chainable, we could address that in a larger pull request. 


@TheAlphaNerd