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.
@kylog Ah good point, and that must be why uptime_minutes wasn't already a fact :). I'll remove that key