Configuration for ie executable.

Component user need to configure the path to graphviz binaries.

This PR adds an unsecure version as a starting point. Without this I cannot generate a SVG output for my Drupal module

```php
$gviz = new Fhaculty\Graph\GraphViz($graph, array(
    'executable' => '/usr/local/bin/dot'
  )
);
$gviz->setFormat('svg');
return $gviz->createImageHtml();
```