[VCARDDAV] vCard revision - AN (anniversary)

[Sly Gryphon <[e-mail]>]

Additional attribute proposal for AN (anniversary), similar to BDAY and
DDAY.

Rationale: Probably the next most important date for contact information
after BDAY -- you could even argue it is more important than DDAY (DDAY
becomes kind of redundant in most cases). Synergises well with a a
RELATED type=partner (i.e. spouse).

AN

   Purpose:  The date of marriage, or equivalent, of the contact
represented by the directory entry.  The value is usually only specified
to the calendar day, without a time component.

   Value type:  The default is a single date value.  It can also be
      reset to a single date-time or text value.

   Cardinality:  (0,1)

   ABNF:

                   param = "VALUE=" ("date" / "date-time" / "text")
                   value = date / date-time / text
                   ; Value and parameter MUST match.

                   param =/ any-param