Add mutation observer

`setImmediate` does not have the recursive blocking loop semantic of node 0.10 [process.nextTick](http://nodejs.org/api/process.html#process_process_nexttick_callback)

Whether or not this event loop starving functionality is good or bad is besides the point. If you want starving use `require('next-tick')`. If you don't want starving use `require('setimmediate')`

With this addition `next-tick` will starve the event loop in node 0.10 and any browsers that support [MutationObserver](http://caniuse.com/mutationobserver)