Prevent duplicate output for commands. Closes #374

If a users decided to use `run_simple`, a command is stopped twice:

1. After the configured wait time
2. On the end of test suite via terminate_all_commands

This makes the output appear twice. This is fixed by this PR. It also
make a method call fail:

1. Command already stopped, `#stop` is called again
2. Comannd has not been started, but ist `#stop`ped

lib/aruba/command.rb

@@ -48,6 +48,8 @@ def initialize(command, opts = {})
 
     # Stop command
     def stop(*)
+      fail Aruba::UserError, 'Please start a command first, before stopping it' if __getobj__.stopped?
+
       __getobj__.stop
       event_bus.notify Events::CommandStopped.new(self)
 
@@ -56,6 +58,8 @@ def stop(*)
 
     # Terminate command
     def terminate(*)
+      return if __getobj__.stopped?
+
       __getobj__.terminate
       event_bus.notify Events::CommandStopped.new(self)
 
@@ -64,6 +68,8 @@ def terminate(*)
 
     # Start command
     def start
+      fail Aruba::UserError, 'Please stop a command first, before starting it' if __getobj__.started?
+
       __getobj__.start
       event_bus.notify Events::CommandStarted.new(self)