added listen grammar

ps-grammar/demos/listen.jsgf

+#JSGF V1.0;
+grammar commands;
+
+public <command> = [tobi] ( <goto> | <fetch> );
+
+<goto> = (go to <location>);
+
+<fetch> = (bring me <objects>)
+
+<location> = (table <numberOfTable>) | shelf | drinks | snacks;
+<numberOfTable> = (one | two | three | four | five | six | seven | eight | nine);
+
+<objects> = (<drinks> | <snacks>);
+<drinks> = (orange juice) | beer | cola | (chocolate milk) | (energy drink) | (grape juice) | milk | water;
+<snacks> = (chocolate cookies) | chips | biscuits | bread | pringles | (strawberry cookies);
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