Test: add dev requirements file & update tox config

- add dev-requirements.txt file
- config tox to install dependencies from dev-requirements.txt file

dev-requirements.txt

+https://hg.logilab.org/master/cubes/geocoding/archive/default.tar.bz2
+https://hg.logilab.org/master/cubes/addressbook/archive/default.tar.bz2

tox.ini

@@ -4,6 +4,7 @@ envlist = py27, py3, flake8
 [testenv]
 deps =
   pytest
+  -rdev-requirements.txt
 commands =
   {envpython} -m pytest {posargs}