Adds support for the VASTAdTagURL-Tag

This tag was part of an older version of the VAST2.0 standard (when it was still in draft, presumably), but it is used on some systems out there. It was replaced by the already supported VASTAdTagURI-Tag. For more information see here: http://support.brightcove.com/en/video-cloud/docs/vast-wrapper-ads
Added testcase
Merge pull request #62 from simpleTechs/support-vastadtagurl

Add support for the VASTAdTagURL-Tag