Fix --ctime-from-mtime accuracy.

Use struct stat's st_mtim instead of st_mtime when available.

Fixes #16
Released 1.12.4.
Fix caching of group members
Merge pull request #18 from alajovic/master

Fix caching of group members
Released 1.12.5.
Implemented rate limiter.

Fixes #12.
Released 1.12.6.