ObserverMutation memory leak in Chrome 

The Problem
---------------------------

A [Chrome GC issue](http://code.google.com/p/chromium/issues/detail?id=160985
) caused the use MutationObservers to hold onto the global context [leaking the entire document between page reloads](http://jsfiddle.net/yjgng/). 

This was discovered when @Megatron3W noticed some serious leaking between reloads in an ember application. https://github.com/emberjs/ember.js/issues/1667

Solution
-----------

Disconnect and null out fhe observer on```window.onunload```

Other solutions where mentioned in the [google code discussion](http://code.google.com/p/chromium/issues/detail?id=160985), but  I[ was unable to make them work](http://jsfiddle.net/yjgng/3/). 

Affected Browsers:
-------------------------

Past and current Chrome (but fixed in dev channel build Chrome 25)