Add better support for parsing vCard 2.1 files

Adds better support for parsing old vCard 2.1 files (#1).  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 decoded
* Supports 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.