Improve support for embedding multiple instances of Flambe

- HtmlExternal now binds functions to the embed target, rather than `window`.  This is more
  consistent with the way it works for Flash and also allows interaction with multiple instances of
  HTML5 flambe.

- `flambe.embed` can take a callback that is fired when embedding is complete.  This could be used,
  for example, to embed multiple flambe instances in sequence.