Added support for greater configuration of unix_http_server

Added three params to supervisor class

    unix_server_file    (default: /var/run/supervisor.sock)
    unix_server_chmod   (default: undef)
    unix_server_chown   (default: undef)

Example of use:

    class {'supervisor':
      unix_server_chmod => '0770',
      unix_server_chown => 'root:admin',
    }

Users belonging to 'admin' group are then able to interact with supervised
processes without resorting to sudo.