+ 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 ...

It doesn't do anything (in terms of fixing an issue), but it should be more correct than what was there previously

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

It was useful for debugging though, maybe an explicit type assertions would
help others ?
As 'others' will be gitpython, I suppose I can handle it myself

It's a nice way of saying that there is still one failing, consistently.

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