Add structured uptime fact

This commit adds a new system_'uptime' structured fact, which
includes keys for seconds, minutes, 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.
Does it make sense to have a `minutes` member of the hash? The other integer members of hash are the total value for seconds/hours/days, whereas the minutes is the minutes relative to the hour. That's useful for formatting the string version of uptime, but I don't see the use case for adding that as a field in the structured fact.