Added addFilesAs function for adding files while changing their paths.

The patch adds an `addFilesAs` function, which takes a `FilePath -> FilePath` function. The main use case is when dealing with absolute paths, so that you don't end up duplicating the whole file system hierarchy inside the archive. `addFiles` was redefined as `addFilesAs id`. The test cases were adjusted so that both `addFiles` and `addFilesAs` are covered.