Add "More details at" link for those layers with a url pointer

Examples:

  https://kurma-earthworks-dev.stanford.edu/catalog/stanford-cg357zz0321
  https://kurma-earthworks-dev.stanford.edu/catalog/mit-001692431

fixes #163

![screen shot 2014-12-05 at 11 05 20 am](https://cloud.githubusercontent.com/assets/1861171/5321286/a615d6b0-7c6e-11e4-915b-a1cacb298df0.png)

This could be shortened to:

``` 
<% if document.references.url.present? %>
```