Use ctime timestamps instead of mtime

Use the last modification time of the inode (metadata), instead of modification time of the file itself. 

This makes sure that files only get deleted x hours after they have been tagged with the self-destruct tag, basically fixing this:

```
WARNING: If you tag a file that was modified in the past, the time-to-delete will be in the past and it may be deleted on the next run of Self Destruct.
```

(note: changing the file's mtime will also update the ctime, so modifying the file will still delay deletion)