print Calendar chanded

- returns `invisible(x)`
- shows `weekdays`
Warn bad setup

- if someone defines holidays without defining weekdays a warn is raised.
Docs updated and dib defaults to NULL

- Calendar's dib argument defaults to NULL
- Docs updated
DESCRIPTION Updated
Calendar's name default value changed

- Calendar's name argument defaults to NULL
- default.calendar was set to Calendar(name='Actual', dib=365), which represents a Actual/365 daycount rule
- bizyears raises an error if dib is NULL