Add the possibility to change the pdf url

Add a watcher on the `url` attribute so we can reload the pdf seamessly once the
pdf url is changed without any particular action to do.

Thus, we can just change the pdf URL in our controller, and the new PDF
will load by itself. For example:

```js
$scope.pdfUrl = '/some/path/to/pdf';

$scope.changePdf = function() {
    $scope.pdfUrl = '/some/path/to/another/pdf';
    // The PDF will load by itself in the next digest
};
```