Add native_utf8, fix _load, clean, is_ascii

UTF8::native_utf8(): added to preserve purity of class file

UTF8::fix _load(): need use require_once because UTF8::$called is public property:
~~~
unset(UTF8::$called['strlen']);
UTF8::strlen($str);
~~~

UTF8::fix _load(): reordered "native_utf8" and "is_ansii" - basic condition "native_utf8" = true.

UTF8::is_ascii(): added support for traversable objects