Start separating the image subsystem

This is a first step towards moving all code related to local
manipulation of images into a cleanly separated subsystem,
accessible via a stable set of commands in the engine API.

`graph.TagStore` now implements `engine.Installer`. For now, it
is installed by `Server.InitServer`, along with all other Server
commands. However this will change in future patches.

`graph.TagStore.Install` registers the following commands:

* `image_set` creates a new image and stores it locally.
* `image_get` returns information about an image stored locally.
* `image_tag` assigns a new name and tag to an existing image.

These commands are a pre-requisite for moving 'push' and 'pull'
out of `Server`.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
beam/data: Message.GetOne() returns the last value set at a key

This is a convenience for callers which are only interested in one value
per key. Similar to how HTTP headers allow multiple keys per value, but
are often used to store and retrieve only one value.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: Env.MultiMap, Env.InitMultiMap: import/export to other formats

* `Env.MultiMap` returns the contents of an Env as `map[string][]string`
* `Env.InitMultiMap` initializes the contents of an Env from a `map[string][]string`

This makes it easier to import and export an Env to other formats
(specifically `beam/data` messages)

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: Receiver and Sender preserve Job.Env

When sending a new job to a `engine.Sender`, the corresponding
`engine.Receiver` will receive that job with its environment preserved.
Previously the job name, arguments and streams were preserved but the
env was lost.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
updated documentation for mentioning images vs containers

Docker-DCO-1.1-Signed-off-by: Robin Speekenbrink <robin@kingsquare.nl> (github: fruitl00p)