Avoid date()/gmdate() for date formatting

Added a MWTimestamp::format method to format a timestamp object in the
given pattern. This avoids date(), which needs the local time
zone corretly set on the server, which is assumed at the moment in
Setup.php

Added MWTimestamp::getInstance for gmdate() and
MWTimestamp::getLocalInstance for date() to create a MWTimestamp object
and use it as inline statement.
Also MWTimestamp::setTimezone and MWTimestamp::getTimzone was added for
timezone handling.

Change-Id: I812aa013be2f4380e0cf10dc465202756fe8347b