Tentative Utf8 patch

This seems to solve some unicode issues I've been hitting. I believe that ```string.length()``` returns the string's length in bytes, and that ```string``` won't be destroyed before the ```gumbo_parse_with_options``` call so the ```strcpy()``` is not needed.

I should point out though that my c++ is very *very* rusty and I only have like two days of node.js experience so you might want to give it a thorough review before hitting that merge button...