New function `setup_package()` that works like `create()` but assumes an existing, not necessarily empty, directory
need to export
rename setup_package to setup
use tryCatch instead of double on.exit
fix indentation
refactor: extract functions extract_package_name and check_package_name, call the latter in create instead of tryCatch
check name validity also in create_description
create_description now uses '.' as default for the path argument
Merge pull request #637 from krlmlr/627-setup-package

New function `setup()` that works like `create()` but assumes an existing, not necessarily empty, directory.
Fixes #627.