Refactored according to reviews

After reviews from @rogpeppe, @howbazaar, @mjs and @voidspace y
refactored a large part of how the tar files are tested, and
how the tar files are generated (most of that logic is to be
credited to @rogpeppe).
Now tar/tar.gz are built recursive and support prefixes to be
stripped if necessary (equivalent to -C on gnutar).
Tests check for files inside the tar and for the sum
Backup returns (filename, sum, err) instead of previous (filename,
sumfilename, err).