Adds thumbnails to post listings

![image](https://cloud.githubusercontent.com/assets/8669803/4944141/d240adbc-65fa-11e4-9b0b-c3a39acc8d93.png)

MMMMMM CAKE
You can use `.presence` here, which returns the thing you called `.presence` on if it's present, or `nil` otherwise. So this whole if/else can be:

```ruby
image_url.presence || DEFAULT_URL
```