Match Swagger Specification 2.0

Specification: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#swagger-object-

Releted issues:
* https://github.com/wcandillon/swagger-js-codegen/issues/10
* https://github.com/wcandillon/swagger-js-codegen/pull/13
* https://github.com/wcandillon/swagger-js-codegen/pull/12
* https://github.com/wcandillon/swagger-js-codegen/pull/11