Updated time_server to use first argument as listen port instead of 8000

This is in response to #14 and nodeschool/discussions#30 (also lots of others). This should ensure that the solution displayed more closely matches the student's submission.

This changes the time_server lesson to ask the student to use the port provided by the first argument.
- updated setup.js to use separate ports via submissionArgs/solutionArgs.
- updated solution.js to listen on process.argv[2]
- updated problem.txt to tell the student to use first argument as the port and added a hint regarding portNumber.

Using program arguments matches the patterns in previous lessons completed by the student, which should be less confusing to them than using process.env.PORT (as suggested in discussion). Maybe using proces.env could be introduced in a separate lesson? :smiley: