Add months and years to Timestamp::getHumanTimestamp()

Now if the difference between $now and $then is >= 30 days, "X months
ago" or "X years ago" is returned.

Month is defined as 30 days, but it shouldn't cause problems  in this
case. The "X time ago" messages are approximate anyway.

Change-Id: I228a11e23783484b64c29db94c3607a415fa81a8
Merge "Add months and years to Timestamp::getHumanTimestamp()"