Allow embedding impetus-enabled elements into other impetus-enabled elements

[fortinmike]

Allows embedding impetus-enabled elements into other impetus-enabled elements, such as a small scrollable div inside a larger one, etc. Without this, the parent and child both react to touch events at the same time, which is unexpected. If you see some use to the "propagate events" scenario (I can't think of any), adding an option to configure this should do the job.