[BUGFIX] IfViewHelper throws an exception when Zend Guard Loader's obfuscation is enabled

This change fixes an issue where the method "render()" on object
Tx_Fed_ViewHelpers_IfViewHelper would cause an exception if Zend Guard
Loader is installed and it's obfuscation function is enabled (default).

Fixed by escaping the function names returned by get_defined_functions()
with preg_quote().
For further information please take a look at issue #30 and
http://php.net/manual/en/function.preg-quote.php.

Fixes: #30