Updates PdfTransformer so that imagick is injected

With this update Imagick is injected in the transformer via
Symfony's Service Container.
The check, relative to imagick extension being loaded, is now up
to the user: the service container will complain about Imagick
not found before injecting it in the transformer.

Imagine/Data/Transformer/PdfTransformer.php

@@ -4,21 +4,28 @@
 
 class PdfTransformer
 {
+    /**
+     *
+     * @var \Imagick
+     */
+    private $imagick;
+
+    public function __construct(\Imagick $imagick)
+    {
+        $this->imagick = $imagick;
+    }
+
     public function applyTransform($absolutePath)
     {
         $info = pathinfo($absolutePath);
         if (isset($info['extension']) && strpos(strtolower($info['extension']), 'pdf') !== false) {
-            //Check if Imagick extension is loaded
-            if (!extension_loaded('Imagick'))
-                throw new \RuntimeException ("PHP Imagick extension is not loaded but required by the PdfTransformer");
-
-            //If it doesn't exists extract the first page of the PDF
+            //If it doesn't exists, extract the first page of the PDF
             if (!file_exists("$absolutePath.png")) {
-                $img = new \Imagick($absolutePath.'[0]');
-                $img->setImageFormat('png');
-                $img->writeImages($absolutePath.'.png', true);
+                $this->imagick->readImage($absolutePath.'[0]');
+                $this->imagick->setImageFormat('png');
+                $this->imagick->writeImage("$absolutePath.png");
+                $this->imagick->clear();
             }
-            //finally update $absolutePath
             $absolutePath .= '.png';
         }
         return $absolutePath;

README.md

@@ -406,16 +406,20 @@ to `Liip\ImagineBundle\Imagine\Data\Transformer\PdfTransformer` as an example.
 
 ExtendedFileSystemLoader extends FileSystemLoader and takes, as argument, an array of transformers.
 In the example, when a file with the pdf extension is passed to the data loader,
-PdfTransformer uses php imagick extension to extract the first page of the document
-and returns it to the data loader as a png image.
+PdfTransformer uses a php imagick object (injected via the service container) 
+to extract the first page of the document and returns it to the data loader as a png image.
 
 To tell the bundle about the transformers, you have to register them as services
 with the new loader:
 
 ```yml
 services:
+    imagick_object:
+        class:   Imagick
     acme_custom_transformer:
         class:     Acme\ImagineBundle\Imagine\Data\Transformer\MyCustomTransformer
+        arguments:
+            -    '@imagick_object'
     custom_loader:
         class:     Acme\ImagineBundle\Imagine\Data\Loader\MyCustomDataLoader
         tags: