new: dev: added a complete reference test.
new: dev: reverse ``natural`` order to get reverse chronological order by default. !refactor

This decision is an effort to get closer to GIT tools philosophy and
implementation. It will allow to stick closer to streams of data from
git commands, and these command should run faster.

To be noted that this particular diffset doesn't bring noticeable
performance improvements.
chg: dev: refactored out the formatting characters from GIT.

This is to prepare to clean __init__.py from any initilisation process.
This will path the way to permit GitCommit being initilised with full
values in the ``git log`` scenario coming in a few commits.
chg: dev: code optimization.

To be noted, we don't seem to need more than identifiers in tags. The
upcoming ``__getattr__`` implementation in GitCommit will remove
useless ``git show`` computations for these commits it seems.
chg: dev: moved commit's attribute retrieval out of ``__init__`` towards ``__getattr__``.

This allows to fetch attribute on demand. This won't have any performance hit yet as
all GitCommit will trigger the full fetch at least once for now.

But we now can initialize the GitCommit attributes with other means. And if we don't use
an attribute that wasn't computed, we will spare a ``git show`` call.
new: much greater performance on big repository by issuing only one shell command for all the commits. (fixes #7)
new: python3 compatibility.
new: pkg: added ``.travis.yml`` for automated tests.