attrib alias support; consolidated build attrib code under Base
support for user_info items that have multiple values
string attributes now return empty strings instead of nil
removed default attrib type
added user_info_lookup attrib handler
warn if user_info_lookup options aren't given
added asin attribute
added musicbrainz attributes
fixed issue #1; added attempt to fallback to year-only parse

The issue was fixed by replacing days/months which were zero and setting them to one
use stricter regexp (Utilities#get_datetime); fixes #2
added read_default handler

formats that don't support a given method now return the default value instead of nil
added audio property attributes
added conductor and remixer attributes
added mood attribute
bump to 0.4.0
Merge branch 'develop'