Light refactoring in groupByElementSet().
Adds and implements a getElementSetName method on the facets model.
Formatting love in the plugin manager class.
Renames getElementsGroupedBySet -> getFacetsGroupedBySet.
Adds findByName and findByElement methods to facet model.
Adds and implements setElementSearchable.
In pingSolrServer, deleteAll, and indexAll helpers, give options parameter a default of array().