GN database is now synced with layer style information, refs #601

This PR will sync GN database with information on styles from GS, will display all the styles associated to a layer in the layer info page, and will give the user the possibility to display each style in the layer info page map.
Please review my code if there is a better approach to this (I am sure there is, I mean there must be a better way to handle this rather than adding an entry point at proxy/views.py, but it will require some GeoExplorer effort, see in #601 comments).