Merge pull request #113 from erikh/network-extensions

simplebridge: only create network/<network>/bridgeInterface once, also
Core continued

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Merge pull request #114 from icecrime/extensions

Core continued
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)