Added new MWTimestamp::getRelativeTimestamp for pure relative.

Since MWTimestamp::getHumanTimestamp just makes a pretty
timestamp, and not a purely relative one, this commit adds
MWTimestamp::getRelativeTimestamp, which does just that.

Change-Id: I8b3e3fc6eeb4afd58f85fa7d05b5ea1a51b0afb6
Merge "Added new MWTimestamp::getRelativeTimestamp for pure relative."