(FACT-612) Add structured uptime fact

This commit adds a new 'system_uptime' structured fact, which
includes keys for seconds, hours, days and uptime. Also,
the existing uptime facts have been modified to use this hash rather
than calling out to Facter::Util::Uptime utilities to generate their
values.
Crud, sorry to keep doing this to you, but noticed one more thing: it's not called `uptime_hash` anymore, it's called `system_uptime` so clearer to update the comments throughout to reflect that.