Improve layout and styling of Curation: Facets page. Fixes #598

Styling changes primarily involved removing the min-height on the panels and adding row and column classes for better alignment, making the list of facet panels more compact and easier to scan.

The other change involves making the display of the distinct values and associated tooltip for a facet conditional, only showing those elements when that facet actually has at least one value.

Before:

![curation_-_search_facets___default_exhibit_-_blacklight](https://cloud.githubusercontent.com/assets/101482/2583988/8f16aaba-b9d8-11e3-9e12-8b0d9cdc3d95.png)

After:

![curation_-_search_facets___default_exhibit_-_blacklight 2](https://cloud.githubusercontent.com/assets/101482/2583989/9ae35460-b9d8-11e3-955b-44b33182bdff.png)