Re-fix Python layout

This allows RBDyn to conform with the usual Python layout on Linux, i.e. the whole `site-packages` vs `dist-packages` conflict. This also fixes things for systems with multiple Python versions (***cough*** Arch ***cough*** and probably the next Ubuntu release). FYI, this is also how things are done in ROS.

We should probably explain how to install RBDyn in the README to have a smooth transition.

TL;DR: went "Bussy" on Python bindings, wait for README update.

cc @haudren @jovenagravante