Fix for bug #546, added docs for event listeners on the host element.

added section on imperatively declared event listeners
mem
I'm not sure you need this last sentence. The important bit that we should mention is that `eventDelegates` is the imperative equivalent of putting the `on-*` declarative handler on the `<polymer-element>`. It's analogous to the `attributes` attribute and the `publish` block. Can we mention this?