xss_clean() improvement

Fixes this: https://github.com/EllisLab/CodeIgniter/issues/2667#issuecomment-37819186
Another xss_clean() improvement

Fixes this: https://github.com/EllisLab/CodeIgniter/issues/2667#issuecomment-37958136
More xss_clean() improvements

Issue described in https://github.com/EllisLab/CodeIgniter/issues/2667#issuecomment-37980030
+ a false positive