Weekdays in RRule output corrected

The RRule class used to display the weekdays in a BYDAY clause surrounded by [], eg. [MO] instead of just MO, or [TU,WE] instead of TU,WE. This is not correct according to RFC 5545, so I changed it. Weekdays are now joined with `join ','`, so I also removed `@last` and `you_are_last()`.