Enhancements (localStorage + roundTripTime + offset API) / different calculation

This pull request adds two api methods:
- offset() (returns the offset in ms)
- round_trip() (returns the round trip time in ms)

It calculates the offset in a different place (when receiving each message back) and does only the averaging in `new_time_sync`.

furthermore, i store the last result in localStorage, so we get a sensible starting value for the next run.