Maximize priority of nextTick

Unroll ticks regardless of platform.  Use the highest priority event
queue avaliable on each platform.

This should also address issues with Node's newly non-recursive
nextTick.  #259.

Seems to have a problem with domains on NodeJS 0.8, but not 0.6 or 0.10.
https://travis-ci.org/kriskowal/q/jobs/6231967

Needs cross browser testing.