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`.