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)

Format like this:

```ruby
BuddyRequestSubject.new(
  a: "b",
  c: "d"
)
```