New draft for the flow-shop chapter.

Latest changes according to feedback from @mdibernardo and a completed chapter draft.
Hey @sevas -- never too late! I think the ```compile_solution()``` is a little too involved to be moved before the general approach section, but it definitely could use some preliminary explanation here. I'll add some text to describe what is returned from the compile solution, and in particular what sits at the [-1][-1] slot -- the start time of the very last job in the schedule.

  Cheers