Fix UITouch timestamp to match Apple's implementation.

The Apple docs explicitly say that their timestamp is based off of
-[NSProccessInfo systemUptime].

Please see:
http://developer.apple.com/library/ios/documentation/uikit/reference/UITouch_Class/Reference/Reference.html#//apple_ref/occ/instp/UITouch/timestamp

Using the timeIntervalSinceReferenceDate can cause interaction
with views that use the timestamp, to behave incorrectly while under test.