Add coverage support via coveralls.io

How this works:

  * The first four entries in the travis matrix run tox with the given
    environment just as they did before. These show as separate line items
    in travis, but they don't contribute to coverage measurement.

  * The last (fifth) entry in the travis matrix runs the full suite of tox
    environments in sequence, accumulating coverage measurements over all
    of them.

  * Regarding dependencies, coveralls is installed in the travis-provided
    virtualenv along with tox. However coverage.py is installed inside the
    tox-provided virtualenv since it needs to run on each Python version.