Expression manager: fix/add some date functionality

With these 3 commits I want to fix/enable some datetime functionality in EM. I put them into one pull request because they only work as a package.

Commit 1 is a partial fix for #7224.
em_javascript.js: LEMval() now returns answers to date questions in yyyy-mm-dd HH:MM format, independent of the date format that is chosen for the question or the particular survey. Thus, comparisons between different date questions are possible (e.g. relevance or validation equations "arrivaldate > departuredate") even when one question is in dd.mm.yy and the other in mm/dd/yyyy HH:MM format (same or previous page).
qanda_helper.php: min/max datepicker logic had to be changed accordingly and is less complicated now.

Commit 2 (#8524) implements min/max date in EM (so dates are validated and the correct help messages appear). Adv. attributes min/maxdate in date/time question supports yyyy, yyyy-mm-dd, questioncode (another date question) and expressions (eg. date(Y-m-d H:i, strtotime(birthdate)+60*60*24*365*18).

Commit 3 is a fix for #8484
qanda_helper.php: Now, expressions in adv. attribute min/maxdate also work for the date picker if variables referred to in the expression are on the same page.

![image](https://f.cloud.github.com/assets/4358578/1883571/4973c122-7988-11e3-8159-4b3b2395a751.png)