errorresponses -> statusCodes, top-level resource DOC element

## Changes

1. rename SwaggerOperation.errorResponses -> SwaggerOperation.statusCodes.
  * Encourages use of statusCodes for all possible status codes returned from an operation.

2. support for top-level resource DOC section.
  * If you include SwaggerApi.overview in your api definition, swagger-ui will render the (html-enabled) overview as a DOC section on top of all of the resource paths/operations in the api.

Note: related to swagger.js pull request (here) which provides swagger-client functionality needed for these improvements.

see example here: http://andycmaj.github.io/swagger-sandbox