Add support for cracking Siemens S7 authentication protocol hashes

Add support for cracking Siemens S7 authentication protocol hashes.

Usage:

```
✗ ../run/s7tojohn.py target.pcap > hash

✗ cat hash 
target.pcap:$siemens-s7$1$62e375eb621bba3a7af9fc06f32d952fcedeec7b$fc8c6bfe744ef880cac56f073829c82d1418f8a9

✗ ../run/john hash  
Loaded 1 password hash (Siemens S7 HMAC-SHA-1 [32/64])
123              (target.pcap)
```

Thanks to Frank for the feedback.