This patch review how scripts are executed and modules loaded.

sys.module is preserved from modification by scripts and avoid conflict with
module loaded by scripts

ISException is an Installsystems exception which can handle an exception and
his information (type, value, traceback).

Signed-off-by: Sébastien Luttringer <sebastien.luttringer@smartjog.com>

With the permission of Romain Degez, CTO of Smartjog and Sébastien Luttringer.

Signed-off-by: Sébastien Luttringer <sebastien.luttringer@smartjog.com>

We now use system tar and gzip to compress/decompress payloads

Goals are:
- Speed-up extraction and creation of payloads.
- Allow to use non-python supported compressor like (pbzip2 and xz)
- Remove python2.7.1 tarfile.py module
- Remove python 2.7.1 gzip.py to avoid filename to be in gzip headers
- Remove gzipstream.py module used to stream gzip file

also optimize tarball.getnames

Add tarfile from python 2.7.2

This allow method add in Tarfile to have filter parameter.
This give us filter capa on file to include.
Should be removed when python 2.7 will be the minimum version

Because we roolback to python 2.6 requirment, we need to ship.

Overide real chown method from tarfile which make crazy check about
uid/gid before chowning. This leads to bug when a uid/gid doesn't
exitsts on the running system

This overide as a sexy side effect which allow badly create tarball
(whithout --numeric-owner) to be extracted correctly

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 allow method add in Tarfile to have filter parameter.
This give us filter capa on file to include.
Should be removed when python 2.7 will be the minimum version