add UTF-8 support

This adds support for UTF-8, see the commit message for details.
Please note that this has not received any significant testing yet.

I am also not entirely happy with the function names "set_js_string_to_lua" and "set_lua_string_to_js", but cannot come up with anything better at the moment.

If you have any comments, I'd be happy to modify the pull request accordingly.
Should the pull request include the built files? Should WEBLUA_VERSION be increased?