Split django.newforms into forms, fields, widgets, util. Also moved unit tests from docstrings to a standalone module in tests/regressiontests/forms, to save docstring memory overhead, keep code readable and fit our exisitng convention

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3945 bcc190cf-cafb-0310-a4f2-bffc1f526a37