Relicense to GPLv3+ with wide exception for all Free Software / Open Source projects + Business options.

Nexedi stack is licensed under Free Software licenses with various exceptions
that cover three business cases:

- Free Software
- Proprietary Software
- Rebranding

As long as one intends to develop Free Software based on Nexedi stack, no
license cost is involved. Developing proprietary software based on Nexedi stack
may require a proprietary exception license. Rebranding Nexedi stack is
prohibited unless rebranding license is acquired.

Through this licensing approach, Nexedi expects to encourage Free Software
development without restrictions and at the same time create a framework for
proprietary software to contribute to the long term sustainability of the
Nexedi stack.

Please see https://www.nexedi.com/licensing for details, rationale and options.

Starting from setuptools 19.4, more concrete from the following commit:

    https://github.com/pypa/setuptools/commit/ebc54982

setuptools sorts namespaced packages .__path__ to be in sync with sys.path .
That however breaks for wendelin.core used from in-tree or installed in
development mode, because we are doing tricks in top-level import redirector
(see e870781d "Top-level in-tree import redirector"):

    (z+numpy.v2)kirr@teco:~/tmp/trashme/wendelin.core$ python -c 'import wendelin'
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "wendelin.py", line 39, in <module>
        __import__('pkg_resources').declare_namespace(__name__)
      File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2081, in declare_namespace
        _handle_ns(packageName, path_item)
      File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2026, in _handle_ns
        _rebuild_mod_path(path, packageName, module)
      File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2050, in _rebuild_mod_path
        orig_path.sort(key=position_in_sys_path)
      File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2045, in position_in_sys_path
        return sys_path.index(_normalize_cached(os.sep.join(parts)))
    ValueError: '/home/kirr/tmp/trashme' is not in list

Here wendelin.py added /home/kirr/tmp/trashme/wendelin.core to .__path__ and
setuptools' _handle_ns() wants to order that dir's parent in correspondence
with sys.path, but parent path is not there - oops.

We can workaround the problem, by first not initializing .__path__ and letting

    __import__('pkg_resources').declare_namespace(__name__)

fully handle and initialize it, and only after it is done we make the
correction for wendelin modules located not under .../wendelin.core/wendelin/
but under .../wendelin.core/ .

Importing was tested to work with the fix with both setuptools 20.6.7 and older
setuptools 17.1.1, i.e. here we should not be breaking backward compatibility.

/reported-by @tatuya, @Camata, @Tyagov

/reviewed-on kirr/wendelin.core!1

For working copy checkout we would like to work on the source
right-away, without putting it under additional wendelin/ (and possibly
/core/) subdirectories.

Add in-tree modules redirector which tweaks plain-python and
pkg_resources-based imports so that e.g.

    import wendelin.bigarray

while in-tree resolves to

    bigarray/__init__.py

In installed state, there will be no such redirector, and code will be
installed with conventional structure.

As a first step we want to make sure the code can be used by open-source
projects without a problem.

So GPLv3+ with wide exception for all open-source licenses.