Moves the follow/unfollow event notification to WordPressComAPI

- FollowButton now uses the correct WordPressComAPI rest method
- Events now naturally originate from WordPressComAPI
- NotificationsFollowDetailViewController no longer needs to send toast since it's done
via the event
Merge pull request #1 from beaucollins/removing-followed-and-unfollowed-blog-toast-message-duplication

Moves the follow/unfollow event notification to WordPressComAPI