Use tox
Fix unicode doctest issue in Python3

We no longer use the `unicode` function (which doesn't exist in python3)
but instead adjust the README doctest to pass a unicode string which
makes the doctests pass in 2.*.

Doctests do not play nicely with Python3 and unicode and so they are
only useful when testing in python2.*

Fixes bug introduced in 73c1575 as part of #6.
Add travis target to icon in README