Added 'Working Backwards' exercise.
Added 'Working Backwards' exercise.

nntisapeh/intro_programming,nntisapeh/intro_programming,BadWizard/intro_programming,BadWizard/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,nntisapeh/intro_programming,nntisapeh/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,BadWizard/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,BadWizard/intro_programming