Introduce Ember.uuid

Ember-Handlebars was using `Ember.$.uuid++` in several places, but
that's actually `jQuery.uuid`, which doesn't exist in jQuery > 1.9.
Thus, we borrow from Ember 1.0
(f3de2718f7d43dccfd7f57bb06440c165c30316d) and introduce `Ember.uuid`
and use it wherever we *were* using `Ember.$.uuid`.