Hosts parsing and logging related changes 

We are excited to try out Herd - and are trying to make it more robust / easier to understand logs before using it in prod.

Hosts parsing: 
- can handle duplicates and empty lines

Logging:
- Printing the time stamp with each log statement helps track progress
- Also printing remaining hosts after each host is finished - we faced a problem where one of the hosts (out of ~50) was stuck for a long time, and it took time to infer from logs which one was stuck

Would a timer which prints status every N seconds be better? It might also be possible to access status of each client directly from tracker.

Let me know..