Core networking

Fix default endpoint creation

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Merge pull request #108 from icecrime/extensions

Build + improvements
Clean up some network extension assumptions, flesh out private API.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
More cleanups, a new Endpoint call Network()

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