Simplify the output of 'docker images'

This is a UI change only, but paves the way for future internals changes.

An image is presented to the user like a package: it has a name, and can have multiple versions.

What is internally known as "repository" is presented to the user as "image".
That is internally knows as "tag" is presented to the user as "version".

Layer IDs are still exposed but de-emphasized, and implicitly presented as an implementation detail

Unnamed images are not shown by default. They can still be shown with 'images -a' but this is a
first step towards hiding them altogether as a top-level construct. This
is necessary to implement frequently requested features such as image squashing.

The following terms are deprecated:
* repository
* tag

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)