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