Some release entries in the MusicBrainz database have different formats
for the date field. In particular, a few have only a year value, while
others have a year and month. This commit introduces a new `parse_date`
function that attempts to parse a string into a date using one of the
known formats. The `release_year` function uses this new function to
parse the date and extract the year, but now also returns `None` if the
date could not be parsed or was not present in the record.
Dustin