[cli] cleanup gen_validator output (Fixes #396)

so we can pipe result to a file:

```
tendermint gen_validator > example.json
```

before this we had to cut first 3 lines:

```
tendermint gen_validator | sed 1,3d > example.json
```

cmd/tendermint/gen_validator.go

@@ -8,16 +8,8 @@ import (
 )
 
 func gen_validator() {
-
 	privValidator := types.GenPrivValidator()
 	privValidatorJSONBytes := wire.JSONBytesPretty(privValidator)
-	fmt.Printf(`Generated a new validator!
-Paste the following JSON into your %v file
-
-%v
-
-`,
-		config.GetString("priv_validator_file"),
-		string(privValidatorJSONBytes),
-	)
+	fmt.Printf(`%v
+`, string(privValidatorJSONBytes))
 }