prevent event propagation on scrubber interaction

When interacting with the scrubber and the scrubber was positioned over the book - the page turn event would fire as you interact with the book.

This issue can be viewed currently at http://test.monoclejs.com/test/controls/index.html
Merge pull request #207 from danshultz/scrubber_event_suppression

prevent event propagation on scrubber interaction