Changes to handshake spec

[lukemarsden]

@progrium and I propose changing the spec as documented here: https://github.com/ClusterHQ/docker-plugins/wiki/PluginProtocol

InterestedIn => Implements

Remove Org because it can be deduced from the container image name. etc

[progrium]

There is an overlap of metadata on plugins vs containers. In general, my thoughts:

• We could rely on MAINTAINER from image history / Dockerfile for Author
• We use the image name for Name
• Website is less valuable without a specific constraint (must be docs, must be repo, etc)
• Version is important, but we assume to use image tags for this

Really that makes handshake very simple.

type Handshake struct {
    Implements []string // extension types implemented
}

[lukemarsden]

PR at #14