Expression managaer: implement PHP function strtotime

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 would make life easier because using {mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}
is rather painful!

DEVS: Please have mercy with me if something's not right ;-). this is the first (tiny) pull request in my life.