Add an OpenAPI

[Seon82]

Closes #6
Websocket specification is currently not supported, see this issue.

[BlankParenthesis]

This looks pretty good overall. I noticed a few potential issues however:

• The URI for the default board is incorrect — it should be /boards/default rather than /board/default.
• POST for placing a pixel lacks a request body — it should be JSON with the single key "color".
• The MinimalBoard representation is missing the max_pixels_available property.

These should be fairly trivial to fix and I'd be happy to merge this once they are and add support for more extensions later.

One final thing that I noticed: ValidationError and the corresponding HTTPValidationError don't seem to be used anywhere and aren't part of the spec as far as I know. Perhaps there's some reason for their existence related to OpenAPI but otherwise I think I'd prefer to see those removed since they seem like either a mistake or noise to me.