Test running improvements (flake8, coverage)

- tox only runs pep8 once, in the pep8 environment
  - Now uses flake8 instead of pep8, which is more comprehensive
- travis-ci should collect coverage information in Python 2.7
  - Also sends it to coveralls.io for visualization
- travis-ci (and locally running tox) now accepts / recommends
  parallelism (if pytest-xdist is installed) with
  `tox -- -n <num_cores>`
- travis-ci using wheels because building is too slow