target="blank" should be target="_blank"

I noticed external links entered as citation details to a source were opened in the same window in stead of in a new one. This is caused by the expandUrls function in WT_Filter. Target="blank" should be target="_blank". Also according to the html5 specs: http://www.w3.org/TR/2010/WD-html5-20100624/browsers.html#valid-browsing-context-name-or-keyword