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)


Pluralize. Also, can we make this `resources :likes, only: [:create]` and that will map to `DiveSites::LikesController` or something.

Alternately, route could be `resources :likes, only: [:create], as: "LikedDiveSites"`