Make REST API documentation easier to use using Swagger

Make [REST API](http://wiki.opencog.org/w/REST_API) easier to use using [Swagger](http://petstore.swagger.wordnik.com).

Can be accessed at:
* http://127.0.0.1:5000/api/v1.1/spec.html for human-friendly UI
* http://127.0.0.1:5000/api/v1.1/spec.json for JSON descriptor

![image](https://cloud.githubusercontent.com/assets/24123/3565412/c2746740-0ab8-11e4-84a7-b806b994bd1c.png)

![selection_419](https://cloud.githubusercontent.com/assets/24123/3565211/4bd83c78-0aaf-11e4-8ded-2153053882fe.png)

Additional dependency is needed in `opencog/python/web/api/install_dependencies.sh` :

    pip install flask-restful-swagger

@cosmoharrigan I hope this is acceptable. Thanks! :)

Fixed #927.