cleanedData is no longer deleted when a form is invalid
Updated Sphinx conf to use the ReadTheDocs theme <3
More API documentation and code tidying

- forms.js API should be almost fully documented now
Refactored common Date/Time input code out into DateTimeBaseInput

- Moved Textarea
- Some doc domment tidy-up
Updated Widget API docs
Widget doc tidy.

- Added TODOs to catch up with Django trunk