When setting visit start date, end date is steup automatically for +3 months

Closes #64 (closed)

smash/web/static/js/visit.js

+function visit_dates_begaviour(startDateInput, endDateInput) {
+    $(startDateInput).change(function () {
+        var object = $(this);
+        try {
+            var date = new Date(object.val());
+            date.setMonth(date.getMonth() + 3);
+            $(endDateInput).val(date.toISOString().substring(0, 10));
+        } catch (err) {
+            //there was a problematic date to process
+        }
+    });
+}