Changed typeof(var) === "undefined" to var === undefined.

This is a little shorter and easier to read.
Fixed wrong xpath syntax.

Moved the //a[@href] to the front of the xpath expressions in hope this
will speedup the hint processing a little. Also disallowed to hint
disabled or readonly input fields. Fixed wrong replacements of "$C" in xpath
expressions, which only relaces the first occurrence of the needle in the
xpath string.