Make partial dates in XMP not have the ommitted fields fulled out to 1's (reported...

Make partial dates in XMP not have the ommitted fields fulled out to 1's (reported by AVRS on irc).

Basically, in XMP you can specify partial date (for example 2011-04 for april 2011 with no day).
We were extracting that, and fulling out the day to 01 if it wasn't present (My original resoning when writing
the code was that the nice localized date functions need a full date, but that's a pretty poor rationale for displaying
wrong date). This changes it to just display such year-month things as YYYY:MM.