Don't use "-w", and just set default posargs

This sets the right default report for coverage (it doesn't include the
omitted `honcho.compat`) and allows overriding which tests to run.
Check for PEP8-compatible naming

Install a flake8 plugin to check for PEP8 naming convention
compatibility.

Only one "error" in the code base that I think is an instance of
pep8-naming actually being wrong. While __new__ is not a class method,
it's also not an instance method (it's a static method) and so should
have "cls" as its first arg, not "self".
Compute coverage with the environment python

Calling tox -e coverage will cause tox to create its own python
environment for coverage, and we'll be computing coverage 5 times for
Python 2.7.

Instead (and I know this ends up duplicating things between .travis.yml
and tox.ini, but I really don't think there's a better way) install the
necessary packages and just compute coverage while running the tests.
This should also result in shorter test runs on Travis.
Compute branch coverage when testing