Need Polymer.Base.unlisten to remove the event listener

[frankiefu]

Currently we have Polymer.Base.listen to add an event listener e.g. tap. But we also need Polymer.Base.unlisten to remove the event listener.

[frankiefu]

This is needed for cases like in iron-selector which wants to default activateEvent to tap (https://github.com/PolymerElements/iron-selector/blob/master/iron-selectable.html#L64). But user of iron-selector may choose to have activateEvent set to something else like down, up or ''. We need a way to remove the default tap event.