Create buddy request notifications

https://trello.com/c/GKdAQ5n3/45-notification-center

![image](https://cloud.githubusercontent.com/assets/8669803/5168453/2d1d34aa-73cd-11e4-9d14-769b2177605d.png)

It's the subject for the polymorphic notification; in this case, it's alerting the user that they have received a friend request