Add a backend for Sparql code

[bmyerz]

Backend to support https://github.com/uwescience/raco/pull/404

Add this to branch off of grappa-master-b

[bmyerz]

https://github.com/uwescience/myria-web/tree/sparql

What I did to add SPARQLBackend

• implement raco.catalog.Catalog
• implement backend.Backend
• add an instance of the backend to Application.backends for myria-web to support it
• add an <option> to backend-menu in templates/editor
• add backend name to list of backends in js/editor.js
• add backend name to list of backends in js/main.js
• grappaends/myriaends dispatch for display (and different query execute url)