doc: clarify where to get actool
Merge pull request #292 from jonboulle/doc

doc: clarify where to get actool
pkg/tar: Create a function for single tar file extraction so it can be reused by
different tar extraction functions.
godeps: Add github.com/docker/docker/pkg/system and one dependency to Godeps.
pkg/tar: Restore files mtime and atime.
pkg/tar: Add overwrite option to ExtractTar.
pkg/tar: Add PathWhitelistMap to whitelist files for tar extraction.
pkg/tar: Add ExtractTarFileToBuf function to extract a single file to a byte slice.
cas: Change Index interface to return errors on Marshal and Unmarshal.
cas: Add function to extract the image manifest from an image in the store.
cas: add new manifest index.
cas: test: create a real tar.
cas: Save manifest on WriteAci.
cas: Add GetAci function.
pkg: add acirenderer package.