etcdctl: tighten up output, reorganize README.md

Documentation was far too repetitive, making it a chore to read and
make changes. All commands are now organized by functionality and all
repetitive bits about return values and output are in a generalized
subsections.

etcdctl's output handling was missing a lot of commands. Similarly,
in many cases an output format could be given but fail to report
an error as expected.

etcdctl/ctlv3/command/printer_json.go

@@ -0,0 +1,41 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package command
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+type jsonPrinter struct{ printer }
+
+func newJSONPrinter() printer {
+	return &jsonPrinter{
+		&printerRPC{newPrinterUnsupported("json"), printJSON},
+	}
+}
+
+func (p *jsonPrinter) EndpointStatus(r []epStatus) { printJSON(r) }
+func (p *jsonPrinter) DBStatus(r dbstatus)         { printJSON(r) }
+
+func printJSON(v interface{}) {
+	b, err := json.Marshal(v)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		return
+	}
+	fmt.Println(string(b))
+}