Add compatibility code for Imagick

Imagick support for using the native clone method was added in 3.0.1rc1. Some distributions (such as Ubuntu 10.04LTS) distribute the older version in the packaging systems. When an older version is used, you receive a Fatal error. There is currently no way to disable this fatal error if the Imagick extension is installed.

This patch may resolve #459.

The error recieved:

    PHP Fatal error:  Trying to clone an uncloneable object of class Imagick in lib/dompdf/lib/class.pdf.php on line 4329