use createChildView and set the container when initing Notify

When using ember-notify with ember 1.7.0-beta5 this error is thrown every time we try to show a notification:
`Uncaught Error: Container was not found when looking up a views template. This is most likely due to manually instantiating an Ember.View. See: http://git.io/EKPpnA`

This PR solves the problem.