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.
Idea: Maybe bootstrap.sh should always call `peru` at the end to fetch the dependencies for real. That way we always end up in a consistent state, instead of needing to worry about whether we're in a "pip-fetched" or "peru-fetched" state in our repo. To do that, we'd probably want to pip out dependencies into some temp location, and then call into `bin/peru` with a custom `PYTHONPATH`, circumventing `peru.sh`. What do you think?