Correctly define window/global DS namespace in IE7/8.

In IE7/8, defining DS globally without directly defining it on
window.DS causes it to not appear in the for..in loop that
processes namespaces inside of findNamespaces()

https://github.com/emberjs/ember.js/blob/master/packages/ember-runtime/lib/system/namespace.js#L117

This causes Ember.Namespace.NAMESPACES_BY_ID to contain an undefined
key instead of DS, which leads to issues when resolving across different
namespaces (such as the DS namespace when using DjangoRESTAdapter), or
when generating toString() outputs of the DS namespace or child objects.

This change allows one existing, failing test to pass in IE8:

https://github.com/emberjs/data/blob/c04b8e47c7de8b158d16bd4850744fec796ca05f/packages/ember-data/tests/unit/model_test.js#L57

which fails because the DS part of the expected toString() output
is missing.