Make sure we are consistent with format in DS

@PascalPiche came across this bug while displaying the weekday name: The DS formats the weekday with 'N', which uses ISO-8601 (first date of the week is then Monday as in the US, not Sunday).

There were multiple places where a fix could be applied, but the best solutions we found was to apply the fix in the data-source for two reasons:

1. Field ouput won't change, so less risk to break things
2. The xsl template is agnostic to this change since it was doing string matches

data-sources/data.datetime.php

@@ -75,7 +75,7 @@ public function grab(array &$param_pool=NULL) {
 
 			// Weekdays
 			$storage['weekdays'] = array();
-			$date->modify('last Sunday');
+			$date->modify('last Monday');
 			for($i = 1; $i <= 7; $i++) {
 				$storage['weekdays'][] = $date->getTimestamp();
 				$date->modify('+1 day');