Adds "namespace" attribute for signal customization

I see `polymer-signals` as a very useful inter-element communication tool, but semantically I don't like the idea of universally using a `polymer-signal` event; it's just not very descriptive. This is a rough sketch and probably needs refinement, but this PR provides the ability to give a "namespace" attribute that determines the signal event. Example:

```html
<polymer-signals namespace="app-signal" on-app-signal-foo="{{doSomething}}"></polymer-signals>
```

The default behavior works exactly as it does today, however another change I'd propose (but didn't implement pending feedback) would be to make the event triggered be simply `signal-{{name}}` instead of `polymer-signal-{{name}}`. In combination with namespaces, this would become a nice little bit of semantics:

```html
<polymer-signals namespace="app-signal" on-signal-foo="{{doSomething}}"></polymer-signals>
```

Of course potentially even simpler from a semantic perspective would be to trigger with just `on-foo` but that runs too much risk of event collision. So my questions:

1. Is namespacing something you see as valuable? It allows element authors to scope signals to their own projects while still reusing the Polymer element, so I say yes.
2. Would you be interested in the `on-signal-{{name}}` implementation?