Access result of VIES request

- make runtests.py executable
- Save result in `VATIN` object (codingjoe/django-vies#5)
- Another example in README.md
- Change the message of `ValueError` raised by `VATIN.is_valid()` to something more applicable in the case suds raises a WebFault + test it
- test for bare `VATIN.is_valid()`

Work in progress. Still thinking of a nice way to make the result accessible from the form field...