Add force flag to remove_dir and remove_file

It was not possible to force `#remove_file` to "delete" an not existing file. It just raised an error. I added a `force`-option, so that no error is raised if a file/directory does not exists.

To make `#remove_dir` backward-compatible I set `force` to `true`, because `Dir.rmdir` does not care if a directory exists or not.