Case insensitivity and matching across whitespace

[enricob]

This commit adds the ability to parse feed urls regardless of the case of the tag, attributes, or scheme in absolute urls and allows attributes of a tag to be matched across whitespace.

These changes are independent of my previous pull request so that you can merge everything however you like. I have an "all_fixes" branch in which I've merged all this together for my own use, if you'd rather use that instead.