Added helper method to link valid urls and set the source field to use that method
Merge pull request #247 from KelvinSmithLibrary/external_links

This takes the content from works where the dc:source fiels is a hyperlink and updates the text to be an embedded hyperlink in the HTML. It does this by replacing the curation_concern_attribute_to_html function with one that checks to see if the the content is a link.