bebot demo teaching

ps-grammar/demoChallenge/commands.jsgf

 #JSGF V1.0;
 grammar bebot_demo;
-public <statement> = [tobi] [please] <command> [please] [tobi];
+public <statement> = [tobi] [please] ( <command> | <teach> ) [please] [tobi];
 
 <command> = <clean> | <party>;
 <clean> = clean up | start cleaning;
 <party> = [the] party starts | party beginning;
+<teach> = ( (this is [a | an | the] ) | ( these are [the] ) ) <object>;
+<object> = [other] ( trashbin  );