Display and save GeoRSS feeds as map layers

Pull request for [GNIP 23 - Add GeoRSS Layers such as Picasa and Youtube Feeds](https://github.com/GeoNode/geonode/wiki/GNIP-23---Add-GeoRSS-Layers-such-as-Picasa-and-Youtube-Feeds)

This functionality depends on another pull request for gxp - https://github.com/opengeo/gxp/pull/161 - where most of the changes were made.