Python 2 support was dropped, there is not need to run tests in py2

+ Use environment-markers in requirement files (see
http://stackoverflow.com/a/33451105/548792).

if used as context managers, the parsers will automatically release their file locks.

Conflicts:
	README.md
	requirements.txt
	setup.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.