Application service injection

Reusing container in your smoke tests FTW:

```yml
default:
    suites:
        default:
            type: symfony_bundle
            contexts:
                - Behat\Sf2DemoBundle\Features\Context\FeatureContext:
                    simpleArg: 'string'
                    session:   @session
            bundle: 'BehatSf2DemoBundle'
    extensions:
        Behat\Symfony2Extension: ~
```

```php
<?php

namespace Behat\Sf2DemoBundle\Features\Context;

use Behat\Behat\Context\Context;
use Symfony\Component\HttpFoundation\Session\Session;

class FeatureContext implements Context
{
    public function __construct(Session $session, $simpleArg)
    {
        // $session is your Symfony2 @session
    }
}
```