Added "tag-images" goal to plugin

Added tagging as a "deploy" goal as a means to "release" an image that
can/should be used in a release/production environment. Images are
explicitly listed (referencing their "build" ids), given one or more
tags and optionally "pushed" with those tags.

Updated README.

While here:

- slight bug fix to ImageDescriptor to allow "." and "-" in tags e.g.
"ubuntu:14.04"
- it maven test needs to run "deploy" phase, but actual maven artifact
deployment needs to disabled in each POM.
- slight refactoring using Optional<> for different ImageDescriptor
members for clarity/consistency
Propagate simple-it's mended Dockerfile to other its

(And merge latest upstream)
Unit tests for new tag-mojo related classes.
Updated README to reflect swapping of phases between push and tag.
Added missing copyright in new class file.
Minor fixes around "keeping"

My logic was a little bit jumbled by double negatives, now:

- a built image is "kept" if marked as "keep" or "push"
- stop mojo passes over "kept" built images

Minor refactoring for clarity. Attached name to copyright on new
classes.
Merge pull request #15 from lcoote/master

Added "tag-images" goal to plugin, thanks @lcoote !