make runtests.py executable
Save result in `VATIN` object, plus:

 - Another example in README.md
 - Change the message of `ValueError` raised by `VATIN.is_valid()` if suds raises a WebFault + test
 - test for bare `VATIN.is_valid()`
Merge pull request #6 from jieter/access-reply

Access result of VIES request
v1.0.0