Demo and API docs

##Polymer.IronResizableBehavior

IronResizableBehavior is a behavior that can be used in Polymer elements to coordinate the flow of resize events between "resizers" (elements that control the size or hidden state of their children) and "resizables" (elements that need to be notified when they are resized or un-hidden by their parents in order to take action on their new measurements).

Elements that perform measurement should add the IronResizableBehavior behavior to their element definition and listen for the iron-resize event on themselves. This event will be fired when they become showing after having been hidden, when they are resized explicitly by another resizable, or when the window has been resized.

Note, the iron-resize event is non-bubbling.