Certificates index

This displays list of certificates under `/certificates`.  It's unlinked at the moment, so a user would only find it by knowing the url.

There are a few changes that bring in the dataset_curator into the response set better so that the publisher is available on the certificate now (related to #440).

![image](https://f.cloud.github.com/assets/51385/894114/b9622b52-fab4-11e2-8334-e556edaec22b.png)