- Trusty comes with python3, pip and virtualenv by default, but write
states defensively to install these dependencies as required and require
the pip or virtualenv states as used by other states.
- Homu requires a python3 toolchain, use virtualenv-3.4 and pass python:
  python3 to get the correct setup (including pip3 inside the venv).
- Update the style guide example to use pkg.installed instead of
  pip.installed to avoid muddling it with a require: pip.
Fixes #142.