3.0 - reworking of API, untagged support

This release reworks the API again, to be more explicit about exactly what will be deleted. This addresses some of the issues with the 2.0 API where the "default" behavior was to delete unexpected things.

This also resolves some potential issues with images that had tags with multiple "names", e.g. an image might be "foo:1.1" as well as "bar:2.0". We've also added unit testing, and some support for doing local integration testing using Vagrant.