Added tox configuration

In order to do #1 I would like a good way to test the documentation, the easiest approach for that is using tox, so I've made this pull request to add a tox configuration file. To remove duplication I've changed the travis configuration to simply call tox.

Unfortunately it's not possible to just run tox like `PYTHONPATH=../wherever/pypy/is tox` as this causes a crash at the moment. The workaround I've found relies on the {env:} substitution tox allows in configuration files, unfortunately this means that tox now always has to be invoked with `PYPY=./wherever/pypy/is tox`.