Pass correct format to JS widget

This fixes two issues:

1) The code didn't take into account that self.format will be ignored by
   Django if `USE_L10N=True`, unless a format was specified when constructing
   the Widget. Remedied by moving the code to `render()` because the current
   language is determined at request time, not at object construction time,
   and looking up the format in the same way that `django.forms.widgets.DateTimeInput` does it in its `_format_value()` method.

2) The code couldn't handle date formats that include dots, like
   31.12.2014.

Fixes #1440.