Partially fixes #36. Register elements only when any `polymer-element`'s registered via html imports are all ready. This makes it easier to use polymer with asynchronous script loading libraries. Previously Polymer waited only for elements registered via calls to `Polymer`. Note, due to crbug.com/395686, the script portion of element registration is still used for ordering, but when this is fixed, we'll switch to order based on the declarative portion of registration.