initial monitoring of added/removed related docs

This is a first implementation based on MutationEvents. It works on Chrome and IE10, but not in Firefox 29. MutationEvents are being deprecated and be replaced by MutationObserver interface. Will migrate the implementation to it and keep this one as a fallback for browsers that still don't provide support for. 

@shahabl / @busykai, please review it and check if the direction seems OK for you. There are some TODOs there but almost for sure I'm missing some others ones.