Allow bindings to wire events to functions

[blois]

I think Polymer should consider using the binding system as the standard way to connect functions to events, via syntax such as:

<div on-click='{{doSomething}}'></div>

This allows two things:

• Assuming that the UI is bound to a ModelView-ish thing, it allows the handler of the 'do something' to be that model view, rather than bouncing through some code in the custom element which may be unrelated to the view or model.
• Avoids many of the current bugs with Polymer events, including: Prevent event bubbling in polyfill #296 Event.cancelBubble cannot be used for stopping event propagation in Polymer #275 on-* event delegation (other than on host node) does not work with non-bubbling events #208 improve on-* delegation by introducing control for Polymer-bubbling (as distinct from DOM bubbling) #259

I did a quick proof-of-concept of this at https://gist.github.com/peteblois/6953310

[dfreedm]

This was implemented with #328