Add miner uptime

Note:
status.pi.temp
status.pi.uptime
status.uptime

These only get updated when parameter "all" is not empty
This happens:
* on pageload
* when refresh button is clicked
* when miner uptime==1
It reduces useless traffic and computation between pi and client.

Possibly, the third case causes unexpected results: it's not sure whether a status update is initiated exactly when uptime==1, it could be 0 or 2. The plan was to activate it just after a miner restarted. Will be fixed.