Add better support for parsing vCard 2.1 files

Adds better support for parsing old vCard 2.1 files.  Outlook and several other email clients still produce files in this format, so they're regrettably still fairly common in the wild.

* ADR fields are now supported.
  * LABELs still need to be implemented.
* QUOTED-PRINTABLE values are now correctly identified and interpreted
* Handle vCard 2.1's boolean PREF attribute.
* Instead of discarding unknown attributes, assume that they belong to 'TYPE,' which they usually are in vC 2.1 files.
  * We could do this more intelligently by checking against the table in section 10.3.4 of the RFC.