move Base_getDiffBetweenDateAndNow from erp5_gadget to erp5_base as it is context independent and can be useful in many other situation than gadget (like reports, ...)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34562 20353a03-c40f-0410-a6d1-a30d3c3de9de