setup: check version against git version

we cannot use directly the git version because when building a debian package from a debian source, git will not
be available. However, when git is there, we can check it's the same version.

Also improve the declaration of scripts, and document a better way to install manually.
follow PEP 376 for versioning and use fallback egg package version for detecting it

odemis.__version__ is a string representing the version

also add a fallback mechanism when git is not available (eg: installed via an egg or a debian package), to
use the egg metadata.
config: update .conf file to work system-wide

Now, we have the odemis.conf default to a system-wide install

Also update the scripts to use the python class instead of a fixed path