Polyfill stopImmediatePropagation on Android 2.3, references #871.

This makes `zepto.html` pass completely on Android 2.3, and
fixes these failures:

EventTest#testTriggerHandler: Result of expression 'e.isImmediatePropagationStopped' [undefined] is not a function.
EventTest#testEventsOnPlainObjects: Result of expression 'e.isImmediatePropagationStopped' [undefined] is not a function.