0.8 gestures - #1309

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What did we just add here? And is this in 0.8, or post-0.8?

This is wip gestures system (currently on-tap and on-track are somewhat supported). @azakus will continue to work it next week. My understanding is this is not part of the 0.8 official release. @sorvell can confirm.

FYI - I noticed that the track gesture introduced in this PR breaks touch scrolling. PolymerElements/paper-drawer-panel content cannot be scrolled on mobile devices because the track gesture is calling preventDefault() on touch move here.

Reduction: http://jsbin.com/wocoba/12/edit?html,output vs http://jsbin.com/wocoba/13/edit?html,output

This should be addressed by the complete gestures rewrite in #1486