Update Youtube transformer regex to support protocol relative URLs

I'm currently using Sanitizer for a rails 3.2 application to scrub user submitted HTML coming in via a richtext editor. One notable thing I'm trying to allow is embedding of Youtube videos.

Everything is working as expected, but I noticed the youtube transformer seems to be out of date; it doesn't match protocol relative URLs Youtube is now generating for their embed codes.

I've updated the README and specs with updated regex that will catch protocol relative youtube embed codes in addtion to embed codes over HTTP and HTTPS.