Favor BYDAY/BYMONTHDAY over BYSETPOS

[joshbeckman]

Given:

• Most of the functionality from BYSETPOS can be handled with BYMONTHDAY
• Few other libraries handling RRULE support BYSETPOS
  • Not supported in text display/parsing in rrule.js
  • Not supported by ice_cube in Ruby

This changes the repeat options to favor display/generation of rules without BYSETPOS. This is configurable via an allowBYSETPOS config field.

This allows the user to generate an RRULE and have it output the correct toText when fed into rrule.js.

[kaunglvlv]

Can we get this merged in? I'm also using rrule.js and BYSETPOS has been an issue.