Add verify to nipap-passwd

Add new feature to nipap-passwd to verify if a username and password is correct. The tool will show if the user exists and if the password is correct.

For request https://github.com/SpriteLink/NIPAP/issues/467
Yeah, you are right. The way you wrote this patch is in line with the current argument parsing. I do like #458 and if you remember, the main reason for not merging that was my rather selfish reason of running NIPAP on machines with Python 2.6 which don't support argparse. Those machines are updated to 2.7 these days so I'd actually be fine with moving to argparse like you suggested, including the change in syntax.

For the sake of keeping a clean commit log, keep the arguments like
```
nipap-passwd --test-user USERNAME --password PASSWORD
```
for this PR. Then you could rebase #458 on current master and rewrite --test-user at the same time (using --username) to align with the new syntax.