Fixes #27 : N has been moved out of the CONTACT table to its own subsidiary
table, therefore now supports multiple values and can be handled (mostly) by
the same structured value utility method as org, adr.
In order to accomplish this, had to add predicate to VCard to determine if
a structured value is type-able before saving/loading (see issue discussion
for that and other related changes).
Added FIXMES and opened issues for some other problems uncovered during
process.