Demo and API docs

##Polymer.IronA11yKeysBehavior

Polymer.IronA11yKeysBehavior provides a normalized interface for processing keyboard commands that pertain to WAI-ARIA best practices. The element takes care of browser differences with respect to Keyboard events and uses an expressive syntax to filter key presses.

Use the keyBindings prototype property to express what combination of keys will trigger the event to fire.

Use the key-event-target attribute to set up event handlers on a specific node. The keys-pressed event will fire when one of the key combinations set with the keys property is pressed.