Concept of data was changed to payload.
Payload is not necessary a tarball of a file or a directory, but, directly a file without tarball around.
This is useful have the same md5 on data/payload and win some space during repo adding.
Now data which come with image are represented by class Payload and all operation are located inside. This is cleaner!
Payload also handle orginial information (mtime,uid,gid,md5,size,mode)

This changement require a lot of changement in Repository and RepositoryManager class

This commit change a lot of things in installsystems way of life.
Repository cache is now disabled on access to remote repository done directly.
New database and tarball format

We stop using bzip2, because implemetnation of python 2.7 bz2 is uncompatible
with pbzip2. So tarball created with pbzip2 cannot be uncompressed with python.
So we need to find a speedy and portable compression algorithm. Gzip is the
best candidate