add weekdays option

```ruby
pry(main)> 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]
```