Add services management

cf : #14 
So, to test this, let's do that:

- add monkeypatch fixture (see blogs tests, I'm using it there)
- monkeypatch the process.communicate method, overriding with a custom function that will return each of the strings we are expecting from the normal `process.communicate` call
- check that the whole `call_service` function returns the correct values given those `process.communicate` behaviours
- Using [parametrize](http://pytest.org/latest/parametrize.html) you should be able to have only one test function with only one monkeypatch, but all the needed `process.communicate` behaviours

If you that's not clear enough, let's put that aside and we'll do it together next time I'm in the office :)