Fixed typos and bugs

Bugs: 
UTF8::clean():
check UTF8::$server_utf8 not specified
Error result: http://kohanaframework.org/3.3/guide-api/Kohana ErrorException [Fatal Error]: Call to undefined function mb_convert_encoding().

UTF8::$server_utf8:
Set Kohana_UTF8::$server_utf8 not true, because UTF8 use property as UTF8::$server_utf8
require_once -> require
Use `preg_replace_callback()` in `_ucwords()`

/e modifier is deprecated since PHP 5.4
Merge branch 'patch-3' of https://github.com/WinterSilence/core into 3.4/winter
Merged UTF8::load() from 3.3/develop
More UTF8 fixes

- removed obscure braces on terminating conditions
- removed obscure NULL definition - it’s considered NULL already
- moved UTF8::$server_utf8 setup to UTF8.php - where the class exists