minor #1136 Add docblock for the $app field (JPry)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1136).

Discussion
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Add docblock for the $app field

Add a docblock to the `$app` field variable, to help out IDEs.

There's not much else to say on this one :)

Commits
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33edafe7 Add docblock for the $app field

src/Silex/WebTestCase.php

@@ -21,6 +21,11 @@ use Symfony\Component\HttpKernel\HttpKernel;
  */
 abstract class WebTestCase extends \PHPUnit_Framework_TestCase
 {
+    /**
+     * Application instance.
+     *
+     * @var Application
+     */
     protected $app;
 
     /**