Javascript skin files may not be rendered correctly + Add support for new Javascript mime types

Indeed it seems some browsers/containers return "application/x-javascript" as the mime type for .js files. XWiki's code only renders javascript files having a "test/javascript" mimetype.

In addition, according to http://www.developersdex.com/asp/message.asp?p=2978&r=5600817, the official mime type for javascript is now "application/javascript".

Modify XWiki so that it renders "application/x-javascript", "application/javascript" and "text/javascript" mime types.