refactor use keyToPath to generate path for the key
feat(dir) add dir releated functions
Merge pull request #62 from xiangli-cmu/add_dir_func

Add dir func
feat(response.go) return etcd,raft index and raft term
Merge pull request #65 from xiangli-cmu/add_index

feat(response.go) return etcd,raft index and raft term
feat(error.go) add a not reachable error type
Merge pull request #67 from xiangli-cmu/add_notReachable_errorType

feat(error.go) add a not reachable error type
fix(etcd): remove Node.PrevValue

PrevValue is no longer in the v2 API. Remove it.
Merge pull request #68 from philips/remove-prevvalue

fix(etcd): remove Node.PrevValue
fix add valid http status codes according the change in etcd
Merge pull request #69 from xiangli-cmu/add_valid_status_code

fix add valid http status codes according the change in etcd
fix(response.go) add new http status code to error type
Merge pull request #70 from xiangli-cmu/add_valid_status_code

fix(response.go) add new http status code to error type