Support NTP for timestamp synchronization

[kumar303]

It's hard to get timestamps in sync between clients and servers so the Haw spec officially recommends using an NTP server. In python there is ntplib for this. Let's do this:

1. Make sure mohawk integrates well with ntplib. Maybe there is nothing to do here, I'm not sure.
2. Document the use of NTP loudly so that people can plan their client / server architecture around using it.

[kumar303]

FYI, NTP has a lot of security holes: https://blog.hboeck.de/archives/863-Dont-update-NTP-stop-using-it.html