Compatible with Django 1.7

This was necessary due to the fact that ``init()`` method from ``forms.fields.Field`` takes no "max_length" paramater. Yet, the ``formfield()`` method from ``models.TextField`` for ``RichTextField`` was adding this parameter (check out line 1853 in ``django.db.models.fields.__init__.py``).

Replacing ``forms.fields.Field`` with ``forms.fields.CharField`` just solved it.

Tested with __django-1.7b1__ / __django-1.6__