Add support of WKT geometry to "download" end point.

Currently, "download" endpoint uses the list of coordinates to represent a polygon. It has limitation to support multipolygon. If we use WKT geometry spec (https://en.wikipedia.org/wiki/Well-known_text) is a good way to represent the polygon (filter). 

For Postgis query, you can use ST_GeomFromText function, to create geometry object from WKT string (http://www.postgis.net/docs/ST_GeomFromText.html)