Greasemonkey in Firefox doesn't have the GM_xmlhttpRequest function, so
the script breaks when it tries to call the function. This adds a check
before falling back to Greasemonkey's GM.xmlHttpRequest() if available.
Adds a log helper to easily prefix all output with the plugin name,
improving the console when multiple youtube-related plugins are used.
Also adds a couple extra log lines to make it easier to see when it's
loaded or sending the request in case things don't go quite right.
Dave Shoreman