Add a ``schema`` command to cmd ctrl to generate schema image. This changeset add the new commande and do some refactoring in cwconfig and schema.py to allow the use of CubicWebNoAppConfiguration with CubicWebSchemaLoader.

usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/run/cubicweb
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances