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.

#! /bin/sh -e
#

if [ "$1" = remove -o "$1" = upgrade ]; then
  if which install-docs >/dev/null 2>&1; then
    install-docs -r cubicweb-doc
  fi
fi

#DEBHELPER#

exit 0