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](https://rozhdestvenskiy.ru/dev/gestures/polymer-gestures/samples/image/index.html) but it need https://github.com/Polymer/polymer-gestures/issues/60 to be fixed first.