Improved admin UI for API Keys

Improved admin UI for API Keys

* Make api keys searchable, by key or username
* Displays more information on the initial screen, with sortable columns
* Use raw_id_fields for user, because detail page can get very slow when there are a lot of api keys

![api_key_admin](https://cloud.githubusercontent.com/assets/142708/5519873/adf2d9e6-891b-11e4-9b82-d7e8a436f6ec.png)
![raw id field](https://cloud.githubusercontent.com/assets/142708/5519874/adf6c0ec-891b-11e4-9deb-c6562be08793.png)