Add UrlDatastream class
Store absolute_location in a UrlDatastream
Change UrlDatastream#url to UrlDatastream#location so as not to clobber ActiveFedora::Datastream#url
Merge pull request #201 from avalonmediasystem/feature/vov-1637-1638-improved

Move MasterFile and Derivative absolute_location out of descMetadata to a dedicate plaintext datastream