Event bubbling can be disabled for binary state components

[vkkostin]

Implements new mixin supports-bubbles-false, that would disable click event propagation if bubbles=false is passed into the component.

Implements this feature for radio buttons, checkboxes, and switches.

Example implementation (the alert will not show if you specifically click the switch):

  <div onclick="alert('You will not be alerted')">
    {{mdc-switch bubbles=false}}
  </div>

[Kerrick]

I'm not sure why Travis CI is failing -- I was able to check out your branch locally and everything passed, so I'm going to blame Travis and merge it.