#1749, #1754, #1901 - Add debugging message if Zend Guard code obfuscation is enabled.

Zend Guard code obfuscation is enabled by default on many PHP 5.3+ installations,
but we don't use it and Kohana 2.4 is incompatible with it (see Trac tickets).
Rather than crash and/or hack core Kohana code, we can display an error message
to help the end user disable it.