django-cms 3.0 compatibility

I've needed this to use the plugin with dj-cms trunk.

So here are the changes in case you want to merge them (they are supposed not to break the compatibility with the 2.x ;) ):

- do not fail if cms.plugin.text does not exist
- use new plugin/change_form.html template, and fallback to old if
  it does not exist

Thanks!

Yohan