MHX importer: edited rigify connection to work with the new rigifiable rig.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4543 96b784f8-4ae3-dd11-aff7-0030488c597c
MHX importer rigify mode now fails gracefully when rigify is not enabled.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4544 96b784f8-4ae3-dd11-aff7-0030488c597c