remove old documentation / unused interfaces

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
excise core.DID and the core package (because that's all it contained)

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
move extensions/state -> state/gitstate

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #115 from erikh/network-extensions

Cleanup extensions directory