Refactor MWTimestamp::getHumanTimestamp and add hook.

Refactor MWTimestamp::getHumanTimestamp and add hook.

Changed logic in MWTimestamp::getHumanTimestamp so that
all the message and formatting was offloaded into the
Language class, keeping only actual timestamp logic in
the MWTimestamp class.

Also added a hook so extensions can override the
human timestamp format.

Change-Id: Ie667088010e24eb6cb569f9e8e8e2553005223eb