Readelementstatus

This adds support for basic readelementstatus.
This, as well as a heap of other scsi data structures, needs us to be able to decode into hierarical dicts,
for this example we want to decode the bits into individual "element status descriptors" so in order to do so
we need varients of add_result and decode_bits  that can decode into a user provided dict instead of self.result

With this basic support you can not list the status of a medium changer :

$ sudo python mtx.py -f /dev/sg13 status
Data Transfer Element: 1:Empty
      Storage Element: 1:Full :VolumeTag=Diabl2-1                       
      Storage Element: 2:Full :VolumeTag=Diabl2-2      
...