Latest version of it is required to show the issues travis shows as well

GIT_PYTHON_TRACE would actually fail (now) if we debugged archive operations.

Related to #239

More to come, especially when it's about strings

There is more work to do though, as many imports are still incorrect.
Also, there are still print statements

When invoking an environement, one might want to pass extra argument to
the command. That is done in tox by invoking an env and passing the
extra arguments after '--' which are then available as '{posargs}'.

Examples:

  # Reports flake8 error statistics
  tox -eflake8 -- --statistics

  # Only run test_util.py tests, printing a line per test:
  tox -epy27 -- --verbose  git/test/test_util.py

Most people know about pep8 which enforce coding style.  pyflakes goes a
step beyond by analyzing the code.

flake8 is basically a wrapper around both pep8 and pyflakes and comes
with some additional checks.  I find it very useful since you only need
to require one package to have a lot of code issues reported to you.

This patch provides a 'flake8' tox environement to easily install and
run the utility on the code base.  One simply has to:

 tox -eflake8

The env has been added to the default list of environement to have
flake8 run by default.

The repository in its current state does not pass checks but I noticed a
pull request fixing pep8 issues.  We can later easily ensure there is no
regression by adjusting Travis configuration to run this env.

More informations about flake8: https://pypi.python.org/pypi/flake8

tox https://pypi.python.org/pypi/tox is a thin wrapper around virtualenv
which let you craft a fresh python environement to execute command in.

It creates the env with virtualenv, install dependencies, run python
setup.py install in it and then execute whatever command you want it to
do and report status.

To do so I simply:

- listed tests dependencies in test-requirements.txt (which are just
  nose and mock)
- provide a tox.ini file which describe how to install the dependencies
  and execute nosetests
- added the module 'coverage' to the list of test dependencies

To run tests simply:

 pip install tox && tox

That will execute the test command 'nosetests' using python2.6 and then
python 2.7.

The additional env 'cover' can be run using: tox -ecover.