Users can like and unlike a listing

[trello](https://trello.com/c/rIV7ViZ0/19-as-a-user-i-can-like-a-listing)

![image](https://cloud.githubusercontent.com/assets/8655639/5014178/9f582108-6a61-11e4-9b09-f40ba73425b1.png)

same here