[#67] New message system

Removed `futile.logger` from the package and implemented another solution for messages, warnings and errors. See `R/messages.r`.

Todo:
- [x] add roxygen documentation
- ~~add option to show/hide messages and warnings, e.g. `cdb(..., verbose = T)`. Default: `verbose = F`. However, it seems a bit unnecessary to pass the `verbose` argument in each `msg, err, wrn` call, and the messages doesn't really belong to the `cdb` class itself. Might save this for a later release, as it is not obvious how to solve it.~~