utils: added resource_string function
Ship: cache getssh result
SourceImage: reset id after build
Shipment: add backref to self into ships
SourceImage: serialize files to yaml with content
migrated from docopt to click
Bump version: 7.1 → 8.0
added comment about image.getid() meaning
use None instead of '' when Container.id or Image.id is not known
Merge pull request #43 from nikicat/master

migrated from docopt to click