Polyfill for missing Event#stopImmediatePropagation

The stopImmediatePropagation method can be called on the event object inside a listener to stop other listeners, of the same event type and registered on the same node, being called. Normally, stopPropagation only stops the event bubbling to parent elements.

The Android 2 browser has missing support for this wonderful method, so I've extracted this shim from FastClick and provided a standalone implementation in case anyone needs it.