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 correct format there.

2) The code couldn't handle date formats that include dots, like
   31.12.2014.

Fixes #1440.