Can add ships 3 square long, horizontally
Ships will lose lives when hit and sink when they have 0 lives left
Grid square status is changed to 'miss' if user fires at an empty square
Grid square status is changed to 'hit' if a ship is hit
Ships will sink when it has 0 lives left - fixed bug
Refactors Ship and Grid classes and rspec files
Slight refactoring to Ship and Grid classes