First version consistent with CRAN guidelines

Changes

• This version uses Roxygen for documentation. Documents are now included in the source .R files, and .Rd man files are generated by devtools::document("package_path").
• S3 methods now have arguments consistent with the respective generics (print(x,...) and summary(object, ...)
• The survival package was added to the dependency, as its pbc data is used in the example as "medically relevant" example.