First stab at changing super

This changes `_super` to be an omnipresent method on ember objects, and to have a `_nextSuper` that decides what is called next. This allows `_super` to be a noop when it has no parent.

Fixes #3523, but has one failing test I still need to address. Should have similar performance characteristics to the `_super` we know and love today.

![failing test](http://f.cl.ly/items/2X3h2G0u1Z220w0B2p0W/Screen%20Shot%202013-11-02%20at%2012.00.45%20AM.png)

I welcome feedback on this strategy.