add optional version attribute to node which is renderen in `to_str` and adapt comparison via `<=>`
lazily add node version information when traversing the `metadata.rb`s
implement `<=>` based on `@name` only so it is consistent with `hash`
remove puts debugging
add -versions flag and pass it down to image creators
put the responsibility of adding version information to the node finders, not graph creators
remove version from initializer and make it write-only
Merge pull request #3 from tknerr/version-labels

Add version labels to nodes