When the --no-cython option is given, it is no longer an error when
derived files are older than source files.

Error messages have been improved.

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.

This is useful when the source is held in a git repository but this
repository is not an usual kwant repository.

Thanks to Bas Nijholt for reporting this.

* Only extract the version from Git when the top level directory of the Git
  repository coincides with the root directory of the Kwant distribution

* Disregard tags on branches that were merged-in (with Git >= 1.8.4).

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