Code migration

This PR migrates code from rosinstall to wstool. Currently wstool depends on rosinstall for historic reasons.
This PR should be a real pain to review, luckily we have many unit tests, so you could also go ahead and fix any regressions as they occur.

I did not remove rosinstall as a dependency from setup.py because ROS installation instructions for hydro may currently only tell people to install python-wstool, and later rely on the presence of rosinstall for rosbuild packages. Feel free to change the setup.py along with the wiki.