add spellcheck requirements file

sphinxcontrib-spelling only seems to work on python2.x, so it's an
optional set of requirements.
add standalone docs testenv to tox configuration
add pypy to tox envlist
add flake8 check to tox suite
add missing spelling words
PEP8 fixes
set up Travis CI to use tox
fix duplication of tox envs on Travis CI
add enchant library to Travis CI setup
reshuffle testing automation with tox and Travis CI