This PR was squashed before being merged into the 2.0.x-dev branch (closes #1296).

Discussion
----------

Add support for the Guard component to the SecurityServiceProvider

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1260
| License       | MIT
| Doc PR        | included

Services configuration are extracted from the SecurityBundle [`guard.xml`](https://github.com/symfony/symfony/blob/3.0/src/Symfony/Bundle/SecurityBundle/Resources/config/guard.xml) and [`GuardAuthenticationFactory`](https://github.com/symfony/symfony/blob/3.0/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/Factory/GuardAuthenticationFactory.php)

Usage is quite simple, the `guard` type can be configured in the firewall like other ones.

```php
$app['app.authenticator'] = function ($app) {
    return new Authenticator();
};

$app->register(new Silex\Provider\SecurityServiceProvider(), [
    'security.firewalls' => [
        'main' => [
            'pattern' => '^/admin',
            'guard' => [
                'authenticators' => [
                    'app.authenticator'
                ]
            ]
        ]
    ]
]);
```

Commits
-------

4b5ccc9a Add support for the Guard component to the SecurityServiceProvider