Add Ember.setupForTesting.

This makes it somewhat easier to unit-test Ember. It also gives us a
good place to add additional customization (as opposed to advocating
simply setting `Ember.testing = true`).
Fixes a typo in the `Ember.Mixin` documentation.
Merge pull request #4183 from KasperTidemann/master

Fixes a typo in the `Ember.Mixin` documentation.
Merge pull request #4159 from rjackson/add_ember_setupForTesting

Add Ember.setupForTesting.