Debugger support for ReactiveCollection

Added a simple collection debugger proxy for ReactiveCollection so it shows up in the VS debugger like other lists do.

Goes from this...

![Debug view of ReactiveCollection raw](http://i.imgur.com/zvjFZve.png)

... to this!

![Debug view of ReactiveCollection as a list](http://i.imgur.com/AZaE6SY.png)