Toggle _onTap should stop event propagation.

[alexdeefuse]

Hi,

I've been trying to use the paper-drawer-panel inside another paper-drawer-panel.
Everything works well except the toggle tap functionality. When the inner paper-drawer-panel toggle is tapped, both drawer panels react to that event.

What needs to be done is:

_onTap: function(event) {
    var targetElement = Polymer.dom(event).localTarget;
    var isTargetToggleElement = targetElement &&
        this.drawerToggleAttribute &&
        targetElement.hasAttribute(this.drawerToggleAttribute);
    if (isTargetToggleElement) {
        // this should be added.
        e.stopPropagation();
        e.preventDefault();

        this.togglePanel();
    }
},

[blasten]

probably just stopPropagation(). Feel free to send a PR with the unit test. Thanks!