Users can like dive sites

https://trello.com/c/cDSm4ctP/33-users-can-like-dive-sites

![likedivesites](https://cloud.githubusercontent.com/assets/8681076/5112943/8fc9ba24-6ff9-11e4-81d4-9419b6e5f7a5.gif)


Should I change this to just `has_many :likes, through`...