fix(Request.client_accepts): Accept parsing doesn't handle application/*

This patch adds support to the parser for robust accept header handling,
courtesy of mimeparse. Care was taken to make the common case fast.

Also, the tox 'flake8' env. was renamed to 'pep8' for consistency with other
open-source projects, making it possible to use a single text editor
build command script across projects.

Fixes #155