The file kwant/_static_version.py is no longer written upon every
invocation of setup.py.  Instead, a file kwant/_kwant_version.py is
injected into source and binary distributions.

The static version in kwant/_kwant_version.py now takes precedence
before any version from git.  This makes the --no-git option to setup.py
no longer necessary, since the existence of kwant/_kwant_version.py has
the same role.  The motivation behind this change is to allow keeping
Kwant distributions in git repositories that have a different layout
from the canonical Kwant git repository.  This is useful for example for
packaging.

The name of the static version file was changed to avoid problems with
stale files.

The code should be easier to understand.  Configuration through a site.cfg file
is supported.