Add object that represents an endpoint in the hierarchy.

This is an attempt to address wordnik/swagger.js/#36 and
part of wordnik/swagger-ui/#232

It adds a SwaggerEndpoints object that is mainly used for structure.  It does not change the behavior of any of the other classes, just allows a way to further group operations in accordance with the swagger spec.  

See:  https://github.com/wordnik/swagger-core/wiki/API-Declaration#apis