Simpler `stopImmediatePropagation()` implementation

We need this in browsers which do not support it natively, namely
Android 2.3. However, there's no need for the full-blown polyfill.

The downside of not being a full-blown polyfill is that
`stopImmediatePropagation()` won't affect handlers that are not added
via Zepto's event methods; e.g. those added directly with native
`addEventListener`.

References #871
Reverts a1a9dce1f45dcb9d6912b44cd1c980a2f5bea145