Constructing CSV download URL from query
Add basic csv_download API view

Does not yet render CSV
Expects a GET, with a query=… parameter, same as the object that would
be POSTed to the find_societies API
Refactored core search logic to a method, since it is shared by
find_societies and csv_download
Implement CSV Renderer
Hook up CSV Renderer to csv_download
Add Content-Disposition header for file download in csv_download
Merge pull request #145 from NESCent/csv-download

Implement CSV downloads of search results