Refs #56

 - Added timezone setting per website. Added API to fetch the list of supported timezones. UTC Manual offsets are supported even if the PHP doesn't have timezone support built in. 
 - Added default timezone setting that the Super user can set. It will be used to pre-select timezones when creating new websites, or setting the default timezone when adding websites via the API without specifying the timezone.
 - Default timezone set to UTC to all existing websites on update. PHP Default timezone set to UTC in index.php and piwik.php.
 - Removed all usage of mysql date/time functions (which are internally doing timezone conversions based on mysql timezone), now using php generated timestamps (in UTC), or dates manually converted to the website timezone.
 - Altered the few Mysql fields that were defaulting to "Current timestamp", they now default to NULL.
 - Deprecated the field log_visit.server_date and log_conversion.server_date as they are now not used. Must use DATE(visit_last_action_time) instead. Note that the new INDEX on (idsite, visit_last_action_time, config_md5config) will greatly benefit the Live! plugin.
 - Deprecated Piwik_Date->get, must now use Piwik_Date->toString
 - Deprecated the DB adapters getCurrentTimezone() feature, as we now don't rely on the DB timezone. Also removed the warning from the install screen.

git-svn-id: http://dev.piwik.org/svn/trunk@2006 59fd770c-687e-43c8-a1e3-f5a4ff64c105