Allow user to search for agencies

This allows the user to do either a quick search (just entering a keyword and searching through the agency names, zipcodes, and city) OR doing an advanced search with multiple fields.  Any of the fields can be used but they don't have to be. Additional fields will be added later. 
Did not know you could use named items here like `:query` and then pass a hash instead of using `?` all over the place. Very nice.