Expression manager: 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 makes life easier compared to using something like {mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}

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DEVS: thanks for the feedback on my first attempt. Had to start over because of git giving me trouble. 

Tried to implemented T White's suggestions in this new version.

strtotime-js behaves a little differently compared to strtotime-php (think the problem is incomplete implemenation of timezones on part of strtotime-js)
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