Add dynamic Blacklight configuration to support curator-defined fields

- add Spotlight::Catalog mixin for applying dynamic configuration to the
  upstream CatalogController

- add Spotlight::BlacklightConfiguration model to store configuration
  options and merge those options with the upstream configuration

- add exhibit#edit_metadata_fields (fixes #7) for selecting fields to
  display on the show and index pages

![screen shot 2014-01-31 at 11 06 55](https://f.cloud.github.com/assets/111218/2053633/e6ec6354-8aaa-11e3-9be1-b08d1a18d45d.png)

(TODO: use checkbox arrays (#69) instead of multiselects)