Added Tar/Untar functionality

Added the tar package which provides functions to tar and untar
files into and from a io.Reader.
This is basically a convenience wrapper around arhive/tar
which is rather rudimentary.
Corrected legacy strings and reviews

Removed outdated references to backup.
Removed incorrect EOF check for directory listing.
Cleaned tests to avoid interdependency with juju/juju.
Small corrections from review comments

Remove remaining legacy code.
Separating untar file writing to be able to use defer for closing.
Added doc for the tar package
Merge pull request #9 from perrito666/add_tar

Added Tar/Untar functionality