Join clubs and leave clubs; new feed updates on joining and leaving

https://trello.com/c/isGzOdpF/16-as-a-user-i-want-to-be-able-to-join-a-cycling-team-group

![image](https://cloud.githubusercontent.com/assets/8669803/5133319/3e02841c-70d2-11e4-8936-724bc49addd1.png)

![image](https://cloud.githubusercontent.com/assets/8669803/5133321/458a94fe-70d2-11e4-842d-420d93bcab6b.png)
No string interpolation in my controllers! Move that noise into the model. Nice clean method calls in here, that is all!