Initial implementation of timezone support for ERP5.

Add preference for saving user's preferred timezone.
(Note: this requirers installing "pytz":http://pytz.sourceforge.net/ on your machines) 


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17622 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/PropertySheet/Preference.py

@@ -78,5 +78,10 @@ class Preference:
       'type'            : 'tokens',
       'default'         : ['plain','html',],
       'mode'            : 'w'},
+    { 'id'              : 'preferred_time_zone',
+      'description'     : 'Preferred timezone',
+      'type'            : 'string',
+      'default'         : '',
+      'mode'            : 'w'},
   )