Added filters to search from filterable attributes. #6543

* Added a test for filtering and facet tally updation on filtering.
* Filters are applied to the facets as well. There does not seem to be a clean way of applying the facet filter to all the facets at once.
* If an attribute is filterable, additional settings can be configured for it, such as displaying the facet counts, multi-selection of attribute values, etc.
* Refactoring done for filterable_attributes list.
* Added a boolean parameter to prevent faceting on autocomplete request.

For reference: [SO Thread](http://stackoverflow.com/questions/12399326/convert-the-facet-filter-query-into-pyes-format)
return Attribute.search([('filterable', '=', True)])