Fixes #36 You can not filter down by a filter that has already been
applied.
Adding the same filter over and over again was possible.
Added a new context variable "filters" and rename the old context
variable "filters" to selected_facets. Checking in the template
to see if a filter has been applied can be done by checking if
the filter is in the context varaible filters.
Changed doc string for get_selected_facets method.
Removed no longer needed statements from get_selected_facets method.
Added filter |title to field name in the active filters search box.
Merge pull request #37 from openrural/fixes/#36-repeat-querystring

Fix for repeated queryset parameters when applying same facet multiple times