Introducing DAYS_PER_WEEK and HOURS_PER_DAY parameter as to allow using
the duration field in the scope of working hours/days input data (i.e a
day consists of 8h and a week has 5 working days). Behind the scenes all
time information will be stored and treated as timedelta.

New input and ouput format is no longer taking seconds and microseconds
into accound. New format is "3w 5d 4h 30m" or
"3weeks 2days 3hours 2minutes".

All tests have been updated accordingly. Still, test coverage needs to be
optimized.

Using setuptools insteadof distutils as to allow 'setup.py develop'.