signal 'I have a broadcast function'

This allows elements to say whether or not they are broadcasting at all in their definition, using:
```
<element name="...">
  <script type="text/ceci">
    Ceci(this, {
      init: function() {
        ...
      },
      listeners: {
        ...
      },
      broadcast: function(e) {
        // prehook code can go here
        this.emit(e);
        // posthook code can go here
      }
    });
  </script>
</element>
```

This gives all element instantiations a `.broadcast` function property. If present, this element can broadcast (note: *can*, not *does*, which requires that the broadcast function actually calls `this.emit`) and the element will be built with a default broadcast channel. Elements lacking the `.broadcast` function property will have a broadcast channel "false" (`Ceci._emptyChannel`)