Expression managager: implement PHP function strtotime

Facilitates using saved dates/times in expression manager. E.g.
calculation of difference in days between two dates entered in previous
questions of the survey

Using things like "{mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}" in EM is rather painful....strtotime makes life much easier....

DEVS: this is the first (tiny) pull request in my life. Have mercy with me if something's not right ;-)