v0.2.0: Changed version to 0.2.0 to allow pre-releases

A few styling changes:

• print.Calendar returns invisible(x) and shows weekdays
• Calendar raises a warning when holidays is set and weekdays is not
• Calendar's dib and name defaults to NULL
• default.calendar is Calendar(name='Actual', dib=365)
• bizyears raises an error if dib is NULL