fix: some error message weren't written on stderr.
new: dev: separated creation of the data structure and its rendering.

This commit prepares for any templating system to take back the job of rendering.
fix: random cosmetics changes @minor
new: added templating system and examples with ``mustache`` template support for restructured text and markdown output format.
new: doc: added some information on path lookup scheme to find ``gitchangelog.rc`` configuration file.
new: added a ``mako`` output engine with standard ReSTructured text format for reference.
new: pkg: will launch ``./autogen.sh`` automatically. Even if called thanks to ``pip install git+https://github.com/vaab/gitchangelog``.

A hack on 'pure lib' had to be added because data files weren't installed
correctly in case of a distant installation with::

    pip install -v git+https://github.com/vaab/gitchangelog.git@mustache

Resulting with templates not accessible.

Removed setuptools dependency that seemed obsolete and was having bad
side effects.
chg: removed ``pkg`` and ``dev`` commits from default sample changelog output.
fix: very minor spacing adjustement @minor
fix: pkg: BSD sed compatibility layer for ``autogen.sh``.
fix: pkg: avoid looping whenever ``autogen.sh`` fails to modify ``setup.py``.