Utf8String: don't allow string casting

[GoogleCodeExporter]

The __toString() implementation is handy, but makes it really easy to return to 
bad string handling.
http://www.mrclay.org/2010/08/06/php-utf-8-string-class/#comment-109531

Safer would be to have __toString() throw an exception, and force use of 
somthing like getBytes() to return a plain string.

Original issue reported on code.google.com by [email protected] on 16 Apr 2011 at 2:15