Quick fix the invoice filename generation

on the server:

>>> u"export_facture_{:%d%m%Y}.txt".format(today)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: zero length field name in format
>>> today
datetime.date(2013, 11, 20)
>>> u"export_facture_{0}.txt".format(today.strftime("%d%m%Y"))
u'export_facture_20112013.txt'

Fixes #27

This looks like a problem linked with python version, we'll investigate.