Need a mechanism through which to receive snap changes events

[Marcus Tomlinson]

Clients such as the snappy store scope / app, and libsnapd-glib/qt need a mechanism through which to receive snap state change signals from snapd (such as install progress, snap installed / uninstalled, etc.).

During the Devices Sprint it was decided that it would be sufficient enough for ALL events to be broadcasted to ALL subscribers. It'd then be left up to the receiving end to ignore those events it wasn't concerned with.

E.g. An event would simply say something along the lines of: “snap X changed from state A to state B”

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Robert Ancell (robert-ancell)]

A proposed solution would be to use [newline delimited JSON](http://ndjson.org/). This would be simpler for clients to manage rather than something heavier like websockets. An example showing how an install event might be sent:

GET /2.0/events HTTP/1.1

HTTP/1.1 200 OK
Content-Type: application/x-ndjson
(snapd blocks)
(a client installs hello-world)
(snapd returns)
{"snaps":{"hello-world":{"status":"installed"}}