Adding support for 'error_response' tags

Adding support for defining error response codes via the `@error_response` tag.
This will generate (swagger 1.0) `errorResponses` JSON fields for each operation, and a "Response Messages" section will be displayed in the Swagger UI for APIs documented with this tag.

Example in the swagger UI:
![image](https://cloud.githubusercontent.com/assets/569413/3756388/ea017c9e-182d-11e4-9679-5d1a031e67e1.png)
 
You may want to use the third argument `:with_type` or maybe `:with_type_and_title` so that you don't later have to use regex for this simple parse.