New feature: add php-func strtotime() to EM

DEV: (Implemented T White's suggestions in this new version.)

DEV: facilitates the calculation with dates/times in expression manager.
E.g.
DEV: calculation of difference in days between two dates entered in
previous
DEV: questions of the survey

DEV: The function makes life much easier. Instead of
DEV:
{mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}
DEV: one can just use {strtotime(date)}.

DEV: strtotime can deal with a lot of different date/time formats.