introduce cluster store

TODO

- [x] Rename Peer to Machine
- [x] Parse the new space separated bootstrap-config format
- [x] Ensure that the NodeName.ID()'s don't have collisions
- [ ] Generate initial cluster ID from a similar hash of the bootstrap-config
- [ ] Write documentation on static bootstrap

@jonboulle "gopath/src/github.com/coreos/etcd/etcdserver/member.go:25: cannot use binary.BigEndian.Uint64(hash[:8]) (type uint64) as type int64 in assignment"