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 tags.

What is internally known as "repository" is presented to the user as "image".

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.

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