pinch/rotate bubbles, cancelable

Fixes from #49

[x] Since pinch and rotate need to prevent scrolling, the recognizer needs to have defaultActions of none for the pinch and rotate gesture like this: https://github.com/Polymer/polymer-gestures/blob/master/src/track.js#L123-L127
[x] The calls to makeGestureEvent should have bubbles: true, cancelable: true, and _source: "pinch" properties set. I made both events to have source "pinch", is it correct? Just it used only in tests and represents the "file" it came from?
[ ] There should be some sample for rotate too, if only logging the angle.

For third item i'll made separate pull request with sample like this but it need https://github.com/Polymer/polymer-gestures/issues/60 to be fixed first.

googlearchive / polymer-gestures