All 3 gitpython projects stopped using memoryviews, but it's probably
safe to release also user's memviews it after it has been converted to
bytes.

+ chrore(deps): depend on *contextlib2* for `ExitStack` in PY2.
+ refact(util): BREAKING API move consts out of utils.
+ style(pep8): fixe all sources.

Now we deal with memory views as well ...

autopep8 -v -j 8 --max-line-length 120 --in-place --recursive

range in py2 will return a list, which can mean a lot of time and memory
is spent on generating it even though it's just used for iteration.

Simplified implementation of MAXSIZE

There were a few things which were being reused consistently for
compatibility purposes, such as the `buffer`/`memoryview` functions
as well as the `izip` method which needed to be aliased for
Python 3.  The `buffer` function was taken from `smmap` [1] and
reworked slightly to handle the optional third parameter.

This also adds a compatibility file dedicated entirely to encoding
issues, which seem to be the biggest problem.  The main functions
were taken in part from the Django project [2] and rewritten
slightly because our needs are a bit more narrow.

A constants file has been added to consistently handle the
constants which are required for the gitdb project in the core
and the tests.  This is part of a greater plan to reorganize
the `util.py` file included in this project.

This points the async extension back at the original repository
and points it to the latest commit.

[1]: https://github.com/Byron/smmap/blob/1af4b42a2354acbb53c7956d647655922658fd80/smmap/util.py#L20-L26
[2]: https://github.com/django/django/blob/b8d255071ead897cf68120cd2fae7c91326ca2cc/django/utils/encoding.py