Fix Chrome (pre v25) MutationObserver Memory Leak

https://github.com/tildeio/rsvp.js/pull/40
https://github.com/emberjs/ember.js/issues/1667
http://code.google.com/p/chromium/issues/detail?id=160985
Merge pull request #1675 from stefanpenner/fix_rsvp_memory_leak

Fix Chrome (pre v25) MutationObserver Memory Leak