use genpy and genmsg pip packages

[asmodehn]

As soon as they are available

[asmodehn]

see :

• first attempt at relying on setuptools ros/genmsg#68
• first attempt at relying on setuptools ros/genpy#82

[asmodehn]

OR, maybe we could somehow instead use the ubuntu package as a dependency ??

• GOOD : packages already made and mantained, in OS environment (not in ROS)
• BAD : packages not in python ecosystem -> might be tricky to embed in virtualenvs for pure python development

OR, maybe we can embed the code directly where we need it (using submodule in source for example) ?