Add weekdays option

```ruby
Event.last.recurrence.events.map(&:start_at) =>
[Thu, 24 Apr 2014 12:24:00 CST +08:00,
 Fri, 25 Apr 2014 12:24:00 CST +08:00,
 Mon, 28 Apr 2014 12:24:00 CST +08:00]
```