(CFACT-48) Add structured uptime fact

This commit adds a new 'system_uptime' fact,
which returns a map including keys for seconds,
hours, days and a human readable uptime.

Also, the existing uptime facts were updated to
take their values from this new structured fact,
rather than resolving them individually.
Merge pull request #72 from whopper/structured_uptime_fact

(CFACT-48) Add structured uptime fact