Add possibility to specifiy HTTP verb in annotations

Add a "method" parameter to the ApiDoc annotation, allowing
to set the HTTP verb of the method.
Useful when you can't use Routing method requirement.
Uses the specified method in the annotation. If none, uses the
requirements of the route. If none, fallback to `'ANY'`
Example:

```php
<?php
    /*
     * My actions full description
     *
     * @ApiDoc(
     *   method = "POST",
     *   description = "This is a description of your API method",
     *   statusCodes = {
     *      201 = "Returned when successful",
     *      405 = "Returned when HTTP verb isn't supported"
     * })
     *
     * @Route(
     *     name     = "my_route",
     *     pattern  = "/my/url",
     *     defaults = { "_format" = "json" }
     * )
     */
    public function myAction()
```

## Remark

No tests yet, as I can't install the vendors.

```
>php composer.phar update --dev --prefer-dist

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  Problem 1
    - Installation request for friendsofsymfony/rest-bundle dev-master -> satisfiable by friendsofsymfony/rest-bundle dev-master.
    - jms/serializer-bundle 0.9.0 conflicts with friendsofsymfony/rest-bundle dev-master.
    - Installation request for jms/serializer-bundle 0.9.* -> satisfiable by jms/serializer-bundle 0.9.0.