use immediate

the way the queue drains (with the shift) and the fact that listeners are triggered unnecessarily when a drain is in progress can cause a notable hit in performance, the actual difference between the [two perfs here](https://github.com/rvagg/memdown/pull/26#issuecomment-57224045) was using browserify's process.nextTick vs using [immediate](https://github.com/calvinmetcalf/immediate).


This pull just uses immediate but if you don't want to include a dependency I can port over the useful features from immediate over to here.