Docs updated
NAMESPACE updated
bizdays moved to S3 generics

bizdays has been moved to S3 generics to accomodate the use of default
calendar set in bizdays.options (another new structure for holding
options and working pretty much like a singleton)
Furtherly:
. offset has been also migrated to S3 generics
. Calendar's name implemented (as S3 generics)
. tests updated