Add clock exercise

I think we should avoid the calendar module and just do the hour/minutes math inline:
```
minutes_add( Clock, Minutes ) ->
  TotalMinutes = (hm2m(Clock) + Minutes) rem day_minutes(),
  m2hm(TotalMinutes).


day_minutes() ->
  24*60.

hm2m({H,M}) ->
  24*H + M.

m2hm(Mraw) ->
  M = Mraw rem day_minutes(),
  { M div 24, M rem 60}.

minutes_substract( Clock, Minutes) ->
  TotalMinutes = (hm2m(Clock) + day_minutes() - Minutes) rem day_minutes(),
  m2hm(TotalMinutes).
```

Very open to better naming of my functions.