Expression manager: add PHP function strtotime

(Implemented T White's suggestions in this new version.)

facilitates the calculation with dates/times in expression manager. E.g.
calculation of difference in days between two dates entered in previous
questions of the survey

The function makes life much easier. Instead of
{mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}
one can just use {strtotime(date)}.

strtotime can deal with a lot of different date/time formats.