expose gestureHandling so the user can change the map behavior.

[Thkasis]

I needed to change gestureHandling to greedy and noticed it's missing from the list of options in the element. Not a big deal but would be nice to include it.

/*FIXME : ADDED gestureHandling ; wasn't part of the original */
_getMapOptions: function() {
  var mapOptions = {
    zoom: this.zoom,
    tilt: this.noAutoTilt ? 0 : 45,
    mapTypeId: this.mapType,
    disableDefaultUI: this.disableDefaultUi,
    mapTypeControl: !this.disableDefaultUi && !this.disableMapTypeControl,
    streetViewControl: !this.disableDefaultUi && !this.disableStreetViewControl,
    disableDoubleClickZoom: this.disableZoom,
    scrollwheel: !this.disableZoom,
    styles: this.styles,
    maxZoom: Number(this.maxZoom),
    minZoom: Number(this.minZoom),

    gestureHandling: this.gestureHandling
  };

Thanks