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 in the Islandora hook_requirements() hook for the
XSLTProcessor class.  If it isn't found, a message is thrown.

islandora.install

@@ -17,7 +17,7 @@ function islandora_requirements($phase) {
     $requirements['islandora_xsltprocessor']['title'] = $t('Islandora XSLTProcessor Prerequisite');
     $requirements['islandora_xsltprocessor']['value'] = $t('Not installed');
     $link = l($t('PHP XSL extension'), 'http://us2.php.net/manual/en/book.xsl.php', array('attributes' => array('target'=>'_blank')));
-    $requirements['islandora_xsltprocessor']['description'] = $t('The !xsllink is required. Check your installed PHP extensions and php.ini file.', array('!xsllink' => $link)),'error');
+    $requirements['islandora_xsltprocessor']['description'] = $t('The !xsllink is required. Check your installed PHP extensions and php.ini file.', array('!xsllink' => $link));
     $requirements['islandora_xsltprocessor']['severity'] = REQUIREMENT_ERROR;
   }