(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.
Just noticed some more tweaks now that minutes isn't in the hash. This should go away now, as should the `minutes` element in the `required` array below.