Add exercise to Stack module
Add exercise to Stack module

RJFreund/OpenDSA,hosamshahin/OpenDSA,RJFreund/OpenDSA,hosamshahin/OpenDSA,RJFreund/OpenDSA,hosamshahin/OpenDSA,RJFreund/OpenDSA,hosamshahin/OpenDSA,RJFreund/OpenDSA,RJFreund/OpenDSA,hosamshahin/OpenDSA,hosamshahin/OpenDSA,RJFreund/OpenDSA,RJFreund/OpenDSA,hosamshahin/OpenDSA,hosamshahin/OpenDSA,hosamshahin/OpenDSA