tox: add a basic tox.ini config file

tox.ini

+[tox]
+envlist=check-manifest,flake8,py3
+
+[testenv:py3]
+deps =
+  pifpaf
+  nose
+commands =
+  pifpaf run postgresql -- nosetests
+
+[testenv:flake8]
+skip_install = true
+deps =
+  flake8
+commands =
+  {envpython} -m flake8
+
+[testenv:check-manifest]
+skip_install = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest {toxinidir}
+