Implement generate-only option for commands that create txs

The new CLI flag builds an unsigned transaction and writes it to STDOUT.
Likewise, REST clients can now append generate_only=true to a request's
query arguments list and expect a JSON response carrying the unsigned
transaction.

Closes: #966

x/slashing/client/cli/tx.go

@@ -33,7 +33,9 @@ func GetCmdUnjail(cdc *wire.Codec) *cobra.Command {
 			}
 
 			msg := slashing.NewMsgUnjail(sdk.ValAddress(valAddr))
-
+			if cliCtx.GenerateOnly {
+				return utils.PrintUnsignedStdTx(txCtx, cliCtx, []sdk.Msg{msg})
+			}
 			return utils.SendTx(txCtx, cliCtx, []sdk.Msg{msg})
 		},
 	}