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.
Backport DictWriter.writeheader from current Python into 2.6
Fix #29

When doing cancelinvoice.customer = self.customer, we access the
customer object for the first time, causing a new database request.

Since the newly created cancelinvoice is pending, it's added before the
customer is retrieved causing an error (customer_id is required).

Assigning the relationship through the id avoids the database access.