Create parent directories for every file type and not for only regular files.
Merge pull request #275 from sgotti/tarcreateparenteveryfile

pkg/tar: create parent directories for every file type
Create a function for single tar file extraction so it can be reused by
different tar extraction functions.
Add github.com/docker/docker/pkg/system and one dependency to Godeps.
Restore files mtime and atime.