Swagger support

This PR adds ability to produce Swagger-compliant JSON for swagger-ui consumption.

Dependent on https://github.com/nelmio/NelmioApiDocBundle/pull/414 and https://github.com/nelmio/NelmioApiDocBundle/pull/413

This adds additional configuration options under `nelmio_api_doc`, two new `@ApiDoc` annotation properties, and two new routes that users can optionally to their application to serve the JSON files in real-time (this is naturally slow. A `nelmio:swagger:dump` command is in the works.)

Documentation will follow.