Accept directories whose name contains a period (#97)

[youdie006]

Fixes #97.

Problem

createTarball / createZipArchive (and the addDir procs they call) decide whether their argument is a directory by inspecting the filename's extension:

if splitFile(dir).ext.len > 0:
  raise newException(ZippyError, "Error adding dir " & dir & " ... appears to be a file?")

So any real directory whose name contains a period is refused. splitFile("/etc/cron.d").ext == ".d", so common directories like /etc/cron.d, /etc/logrotate.d, network.d are rejected and no archive is created.

Fix

Check the filesystem with dirExists instead of guessing from the name (both tarballs_v1.nim and ziparchives_v1.nim; std/os is already imported). This accepts dotted directory names and still rejects files and nonexistent paths.

Test

Added tests/test_issue_97.nim (registered in tests/all.nim): it creates a directory named zippy_dotted.d, archives it with createTarball and createZipArchive, and round-trips the contents; it also asserts a real file is still rejected with ZippyError. Red-green verified with nim c -r: before the fix createTarball raises ... appears to be a file? on the dotted directory; after, both archive and extract succeed. The full tests/all.nim suite still passes.