calculate hours in addition to minutes and seconds

mpcpl_time.py:
- calculate hours as well as hours and minutes
- change calculate_duration to calculate_duration_units() for more
  clarity
- use an OrderedDict instead of a namedtuple. This is to make easier to
  print only relevant units (e.g., if a playlist duration is less than
  an hour, I don't want to see a printout that says 0 hours)
- pull out the code in main and put them into functions. I decided it
  would be nicer this way as now you can quickly see what's going on in
  main(). Also, great for profiling.