feat(discovery): initial working code

This is an initial version of the bootstrap code that seems to work under
the normal circumstances. I need to mock out a server that will test out
all of the error cases now.

Example use:

```
URL=$(curl -X PUT https://discovery.etcd.io/new)

./bin/etcd  -name cluster1-peer1 -data-dir cluster1-peer1.etcd -discovery ${URL} -addr 127.0.0.1:4001 -peer-addr 127.0.0.1:7001 &
./bin/etcd  -name cluster1-peer2 -data-dir cluster1-peer2.etcd -discovery ${URL} -addr 127.0.0.1:4002 -peer-addr 127.0.0.1:7002 &
./bin/etcd  -name cluster1-peer3 -data-dir cluster1-peer3.etcd -discovery ${URL} -addr 127.0.0.1:4003 -peer-addr 127.0.0.1:7003 &
```