Helpers -- Catch UnicodeDecodeError for tags.

While returning IPTC tags some of the tags might have UTF8 encoded characters directly printed in the string, without the whole string being marked as unicode string. To catch that error, we pass the tag through unicode(source, 'unicode-encode').
Models -- Apply strip() on tags.

To make sure that we don't have leading/trailing whitespace we apply strip to all tags.