Improved JSON responses

[mfojtik]

Hi,

this patches add feature listing to API entrypoints, remove obsoleted
collections and empty attributes (prevent 'something: nil' in response)
and improve reporting of actions in JSON to:

actions: [
{
    start: {
      href: "/api/instances/inst0/start"
      method: "post"
    }
}
{
    destroy: {
      href: "/api/instances/inst0"
      method: "delete"
    }
}
]

  -- Michal