Add utility functions and fixes for iPhone/iPad integration.

This adds some stuff to gravy to make apps work better on the iPhone/iPad.

A new $.proxyTouchEvents function has been added, which causes touch screen
events to be forwarded to existing mouse event handlers. It's intended for
basic use cases where multi-touch events really don't need to be handled.

$.browser has been updated with a 'mobileSafari' check, making it easier to
determine whether we're running on the iPhone/iPad.

A workaround for jQuery bug #6446 has been implemented, which basically fixes
an issue with the result from $.offset() being incorrect.

Tooltips are disabled on iPhone/Pad, as they're really not at all useful and
their hover events manage to get in the way of other events.

Tested this with my Review Board change (/r/1592/).