add consul support as a disovery backend

Signed-off-by: JChien <jeffchien13@gmail.com>
fix readme and remove swarm prefix

Signed-off-by: JChien <jeffchien13@gmail.com>
fix style to match gofmt

Signed-off-by: JChien <jeffchien13@gmail.com>
fix flag usage message

Signed-off-by: JChien <jeffchien13@gmail.com>
use consul's watch

Signed-off-by: JChien <jeffchien13@gmail.com>
update README.md & ROADMAP.md

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge branch 'JeffChien-discovery'