add SAN support base on switch-to-stdlib-csr branch

Solve https://github.com/coreos/etcd-ca/issues/29 by adding SubjectAltName support.
This patch base on https://github.com/coreos/etcd-ca/pull/37
Options -ip and -domain now accept comma seperated ip/domain list
Generated cert is tested against etcd 0.4.6 and work fine