Add overloading functions
Add cdb-class (S4)
Update "["-method
Update readme
Update readme
Update readme
Update documentation
Add constants
Add format attribute to get_variable
Add to-char function
Add na and type functionality to cdb()

"class" is a more correct defintion of the "type" argument. However, it
cannot be used due to class(). "data.class" is a bit long.
Add validation
Add warning when lookup doesn't exists
Add more constants
Fix error
Update documentation
Add tests
Change db_ver bytes 8 -> 4
Update coding