Add nest (multi-dimensional) field name support

Support for nested (multi-dimensional) data in HTML forms.  (Useful if javascript disabled, as graceful option)

To activate, set nested_fields setting to true:
'NESTED_FIELDS': True,

Usage:
Field name:   <input name="animal.dog.dachshund" type="text" />
Accessing:    request.DATA['animal']['dog']['dachshund']

Cheers! :)