Computing workdays

An experimental implementation of an algorithm to compute workdays
between two dates. Holidays aren't taken into account. This
implementation considers saturday and sunday as non-business days. The
next approach is to allow the use of other weekdays as non-business
days.
DayCount's Period conversion methods implemented
Organized the code and simplified some algorithms
Started Period's __str__ method implementation

I need to adjust the string to plural and floating point numbers.
Testing for GenericPeriod.size method call. It must raise an Exception.