First step toward modular discoevery services

Signed-off-by: Victor Vieux <vieux@docker.com>
add file

Signed-off-by: Victor Vieux <vieux@docker.com>
add watch interface
update README.md
update file
allow custom url on token

Signed-off-by: Victor Vieux <vieux@docker.com>
add etcd

Signed-off-by: Victor Vieux <vieux@docker.com>
add discovery/README.md
add node type

Signed-off-by: Victor Vieux <vieux@docker.com>
use 1.5*heartbeat as TTL in etcd

Signed-off-by: Victor Vieux <vieux@docker.com>
Initialize in the interface

Signed-off-by: Victor Vieux <vieux@docker.com>
update READMEs

Signed-off-by: Victor Vieux <vieux@docker.com>