Added new features and some minor fixes

- implemented set_default_workspace
- implemented upload from postgis database
- implemented support for group layers definition using 'publishables' instead of 'layers' (gs > 2.3)
- support for retrieving geoserver information and version number
- Added modules to interact with geonode, gwc, and to retrieve wps processes and settings. These are added in separate modules, but should eventually be merged into the catalog object itself, or at least have some common entry point fro them all, as they all refer to a catalog.