Added day of the week modifier methods. Including firstOfMonth(), lastOfMonth(), nthOfMonth(), next(), last(), and so on.
Fixed up spacing issues. Moved day of week names and associated them with day of week consts.
Fixed up comments and reversed parameter order on nth methods ($nth is not optional).
Fixed up comments and made the $dayOfWeek required for nth methods.
Removed trailing parameter.
Renamed last to previous to avoid confusion between last going backwards and lastOfMonth probably going forward.  I realize people say "last tuesday" but I was initially confused so I think previous makes more sense.
Merge remote-tracking branch 'upstream/master'