generalize polymer's fuoc prevention strategy; fixes: #276
 - styling is now expressed via 'polymer-veiled' and 'polymer-unveil' classes which are now user customizable.
 - to veil (initially hide) an element either include the class 'polymer-veiled' or add to the array: Polymer.veiledElements. This list includes 'body' by default but that can be removed to prevent body from getting de-fuoc'd.
 - note: Polymer automatically unveils elements at WebComponentsReady time, but elements can be dynamically unveiled by (1) adding the polymer-veiled class to them to hide them and then (2) when they are ready, calling Polymer.unveilElements().