Merge pull request #51 from nikicat/master

README.md rewrite, various improvements
clarify if/else logic in Image.getfullrepository
more debug logging when generating Shipment
Container: override __getstate__ instead of __setstate__ to allow recursive save
port specifications replaced with Interfaces
refactoring: added refs to parent to all child objects
fixed --clear-cache handling
added "shipment objgraph" command that dump object graph
fixed tests
added "volumes" command group
added "interfaces" command group
added "ships containers" command group
Bump version: 8.1 → 9.0