Add a method to return the complete list of space permissions

The getPermissions and getPermissionsForUser methods return the list of space-level permissions for the current or specified user.

What we lack is the ability to see all the permissions for a given space. In order to build a complete view, a developer would need to run the getPermissionsForUser method against all users on the site. Ideally, they should simply be able to get the list of permissions (including group permissions) for the site.