Redirect to exhibit home page when the Spotlight::CatalogController#index ...

... has no search parameters.

Fixes #631 

Clicking the "x" here....

![default_exhibit_-_blacklight_search_results](https://cloud.githubusercontent.com/assets/101482/2685063/4a26b57e-c1c3-11e3-935b-3ff09212fa22.png)

... now redirects to this page:

![home-page](https://cloud.githubusercontent.com/assets/96776/2746407/c54c3bf6-c746-11e3-9c6d-8bf2a59fcf4f.png)