Canvas routes

[Marie000]

The basic routes for canvases. POST /canvas PUT /canvas/:id DELETE /canvas/:id GET /canvas/:id GET /canvas

maybe I should have named them 'canvases' but looks like we will be renaming canvases, so I didn't bother putting that much thought into it.

For GET /canvas/:id the Canvas object returned will include, in addition to the normal Canvas properties:

• Reader (you probably don't need this, but I use it in the API)
• Notebook (a canvas belong to a single notebook, so you will be getting the Notebook object)
• NoteContexts (an array of NoteContext objects. These can be outlines, etc.) Same thing with the GET /canvas

For the GET /canvas, I haven't implemented any pagination or filtering. I will add that later.