Add check for PHP XSLT Processor extension.

When the PHP XSLT Processor extension is not loaded, Islandora will silently fail when attempting to add new objects.  This changeset adds a check to the Islandora configuration screen for the XSLTProcessor class.  If it isn't found, a message is thrown.

The "silently fails" happens when trying to move from the metadata entry screen of adding an object to the object upload screen.  The returned page is empty, and this error is sent to the apache error log:

```
PHP Fatal error:  Class 'XSLTProcessor' not found in .../islandora_xml_forms/builder/includes/associations.inc on line 198
```