Zend_Validate_EmailAddress: IDN domains are converted to punnycode if possible

Fixes #62

The problem is that IDN domain cannot be converted to `xn--` in PHP 5.2 (there are some PHP libraries, that implement it, but we can't include them in ZF). So this is rather a "progressive enhacement" for PHP 5.3+ (with intl enabled)

cc @froschdesign