[BUGFIX beta] Only register core libraries once.

In Ember CLI (and in our own test suite) many Ember.Application instances are created.  This prevents each test from triggering a warning that the library has already been registered.

**tldr;** Prevents the following:

![image](https://cloud.githubusercontent.com/assets/12637/5193981/322d247e-74d5-11e4-9bbf-a9d4d3cc44f3.png)