dompdf is too slow; allow the use of wkhtmltopdf instead

See forum post http://forum.civicrm.org/index.php/topic,22626.0.html - on large documents, dompdf can take many minutes, timing out the most generous Apache installation. This patch allows the users to provide a path to wkhtmltopdf; if this option is set, generation of PDFs will use the snappy library (https://github.com/knplabs/snappy) instead of dompdf. (Snappy is a wrapper around wkhtmltopdf).