tarsum: pax header and xattr support

tar archive have additional headers that store file data like security
capabilities and ACLs. This data is over looked when calculating the
TarSum of a layer. Therefore it could be possible to modify the
capabilities of the files inside a layer, and not affect the TarSum of
that layer.

Since python's tarfile does not support these xattr headers, and has
encoding issues on the values stored. This change includes using a
vendored version of python-2.7.6's tarfile.py, with changes to support
this.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)