Make ember-debug an ES6 package.

This is to enable sharing of Ember global setup code, and to prevent a
require issue where Ember.Logger isn't setup properly when Ember.warn is
called.
Ensure that window.Em is setup in ember-metal/core.
Split Ember.copy into a separate module.

This is to avoid circular requires. Ember.copy requires Copyable and
EmberObject.
ES6ify more ember-runtime.