Add a description of the websocket protocol to the spec

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Add an initial implementation of exec over websockets

There are still some terminal/buffering issues when running bash directly
(e.g. tab completion doesn't really work, the terminal gets confused), I will
work on those ASAP. The basics are here, though.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>