Added first version of advanced search

- Added an advanced_controller that builds a bunch of
  nested solr queries based on url params and lets catalog_controller
  handle search.
- Added rendering of advanced search constraints: one constraint per
  nested solr query (or per field if you like).
- There is a lot of quick'n'dirty stuff since this is not a final
  commit of the advanced search - it's more of an inspiration for further
  development. Especially the selection of fields to display on search
  forms and for use in searches should be reworked and maybe tied better
  to the blacklight config.