Ported pinch gesture

Just ported `pinch` gesture from `PointerEvents` repository and added a simple example of it.

I not tested, but maybe it requires those PR to be merged first
https://github.com/Polymer/polymer-gestures/pull/48
https://github.com/Polymer/polymer-gestures/pull/47

At least i had to fix them before i could start porting `pinch`;