package and bootstrap peru

Bootstrap peru by using pip3 to fetch pyyaml and dump it into third-party/. Additionally, a peru build file can be used to fetch pyyaml with a working copy of peru. pyyaml has been removed from the repo.

setup.py has been added so that setuptools can be used to generate Python packages for peru.
Here and in `test.sh`, we call `bootstrap.sh` after sourcing `env.sh`. Maybe we should just make the latter call the former?