tox env to easily run flake8

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

test-requirements.txt

@@ -1,4 +1,5 @@
 # Remember to update README.md
 coverage
+flake8
 nose
 mock

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py26,py27
+envlist = py26,py27,flake8
 
 [testenv]
 commands = nosetests
@@ -9,5 +9,12 @@ deps = -r{toxinidir}/requirements.txt
 [testenv:cover]
 commands = nosetests --with-coverage
 
+[testenv:flake8]
+commands = flake8
+
 [testenv:venv]
 commands = {posargs}
+
+[flake8]
+#show-source = True
+exclude = .tox,.venv,build,dist,doc,git/ext/