Support multiple descriptions per status code

[PR93](https://github.com/nelmio/NelmioApiDocBundle/pull/93) added support for the status codes. This PR just improves it to allow multiple descriptions per status code.

Example:

````php
@ApiDoc(
   statusCodes={
      200="Returned when successful",
      404={
        "Returned when something does not exist",
        "Returned when something else does not exist"
      }
   }
````