Allow to use other annotations or to change behavior of them easily.

Hi,

I would like to use `swagger-php` in my Symfony application to generate documentations.

I have already many annotations to build swagger models and i would like to use them to interact with `swagger-php`.

At this moment, it's impossible to use an other one annotation than predefined annotation or to edit behavior of theses annotations. That's why i purpose to use a `ProcessManager` to implement many `processors` which will create/edit swagger models. By this way, use will be able to interact with any annotation in any context (Class, Method, Property).

For sure, this PR is totally **backward compatible**.

*Example of a Processor on Symfony:*

```php
<?php

namespace Rezzza\SwaggerBundle\Processor;

use Symfony\Component\Routing\Annotation\Route;
use Swagger\Processor\ProcessorInterface;
use Swagger\Parser;

/**
 * RouteProcessor
 *
 * @uses ProcessorInterface
 * @author Stephane PY <py.stephane1@gmail.com>
 */
class RouteProcessor implements ProcessorInterface
{
    /**
     * {@inheritdoc}
     */
    public function supports($annotation, $context)
    {
        return $annotation instanceof Route;
    }

    /**
     * {@inheritdoc}
     */
    public function process(Parser $parser, $annotation, $context)
    {
         // Create a resource object or get current and ...
         // $parser->appendRessource(....);
    }

    /**
     * {@inheritdoc}
     */
    public function getId()
    {
        return 'rezzza_symfony_route';
    }
}
```

If you have any question/suggestion/doubt ... feel free to ask me.

Regards