iOS 8 Interactive Notifications for Comments

Adds actions to comment notifications in iOS 8.

* Unapproved comments will have an 'Approve' action.
* Approved comments will have 'Reply' and 'Like' actions. If likes are disabled on the site it will only show 'Reply'.

Both 'Approve' and 'Like' actions run in the background. 'Reply' will take you to the notification detail view.

Note: The WP.com changes are live for this feature. To test, run this PR on an iOS 8 device and send yourself a comment notification.
I wish NSSet had a literal.