Support for converting ANNIVERSARY

vcard 3 -> 4
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Find all X-ABDATE and X-ANNIVERSARY properties, and merge the ones with identical values. Convert all of these to ANNIVERSARY.

vcard 4 -> 3
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Convert ANNIVERSARY to both X-ABDATE and X-ANNIVERSARY.
oh I am wrong https://github.com/fruux/sabre-vobject/commit/151d53e2d4fd38d0c610dade5653eff65117ce87