extract NTP logic
make NTP pool configurable

use the world-wide pool by default, but specify north-america in
user.yml. Also, documentation. This way Sovereign will still behave the
same, but the NTP servers can be changed when desired.
use the geo located ntp servers for everyone

still allow people to override them in defaults.yml
Merge pull request #138 from tilsammans/ntp

Make the ntp pool configurable