luci-app-statistics: Add initial support for chrony

Add initial support for chrony statistics.
At this point introduce time offset and stratum stats.

There is something strange with the host option.
Anything else than 'localhost' or '127.0.0.1' causes an error at
collectd start, even when /tmp/collectd/conf is directly edited

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>