normalizeDate: Allow dates with year only (%Y)

Where source metadata has dates in the form of 2013, the normalizeDate function destroys them.

Often only they year portion of a dates (say copyright) is known.
dc metadata supports dates "at any level of granularity":
http://dublincore.org/documents/2012/06/14/dcmi-terms/?v=elements#terms-date

ISO 8601 W3CDTF (which is recommended best practice) specifically allows YYYY.
see http://www.w3.org/TR/NOTE-datetime

Ideally year only dates would be left as is; currently they have January 1 added to them to adhere to format %F.