add support for nth occurrence of weekdays

ex: 2MO (second Monday of month)
ex: -1SU (last Sunday of month)

lib/event.js

@@ -309,12 +309,12 @@ var ICalEvent = function(_data, calendar) {
 
             data.repeating.byDay = [];
             repeating.byDay.forEach(function(symbol) {
-                var s = symbol.toString().toUpperCase();
-                if(['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'].indexOf(s) === -1) {
-                    throw '`repeating.byDay` contains invalid value `' + s + '`!';
+                var s = symbol.toString().toUpperCase().match(/^(\d*||-\d+)(\w+)$/);
+                if(['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'].indexOf(s[2]) === -1) {
+                    throw '`repeating.byDay` contains invalid value `' + s[2] + '`!';
                 }
 
-                data.repeating.byDay.push(s);
+                data.repeating.byDay.push(s[1]+s[2]);
             });
         }