shared model support, resource overview support

## Changes

1. rename operation.errorResponses -> operation.statusCodes.
  * Better indicate that people can use this property to represent all status codes returned from an operation
  * Will break swagger-ui without incorporating accompanying change in related pull request.

2. add shared model support
  * Allows definition in one file of shared models that can be referenced from many resources. Avoid having to copy and paste model definitions between resource definitions

3. add resource overviews: allows general documentation overview for an entire resource.


These changes will enable the improvements in the Swagger client. In order to display in swagger-ui, see accompanying pull request in swagger-ui.

See example here: http://andycmaj.github.io/swagger-sandbox.