Fix error when HTMLText / AbstractText are mixed in the same model

Currently ``initCMSCKEditor()`` is called once per page, this makes impossible to add a ``HTMLField`` to an ``AbstractText`` child. Incidentally currently multiple ``HTMLFields`` in the same model works, but you can't apply a different configuration to each.
This is going to solve this as well.

Fix #186 
``{{ ckeditor_selector }}`` is a Django template var, thus I suppose that underscores are more appropriate