Improved process.nextTick

Created a function that uses window.postMessage and falls back to setTimeout, also queues functions so they will all be executed in order on the next tick.

Perf tests:

http://jsperf.com/nexttick-vs-setzerotimeout-vs-settimeout/3
http://jsperf.com/multiple-nexttick-vs-setzerotimeout-vs-settimeout/3