new feature #7811: implement strtotime() for EM

DEV: already implemented in master...now for 2.05
DEV: see pull request https://github.com/LimeSurvey/LimeSurvey/pull/85

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