Unicode fixes for Python 2 (and tests for Python 3)

[henryiii]

Python 2 Plumbum fails (for example, on Travis) if you have unicode in your output. This fixes that, and tests it.

It is still not supported to use a unicode path in Python 2 (and probably that limitation will remain until Python 2 dies).

[coveralls]

Coverage decreased (-0.04%) to 81.073% when pulling 4a0703d on uni into a39e7ba on master.

[coveralls]

Coverage decreased (-0.04%) to 81.073% when pulling dca6d3b on uni into a39e7ba on master.

[coveralls]

Coverage decreased (-0.06%) to 81.051% when pulling db6ccb5 on uni into a39e7ba on master.

[coveralls]

Coverage decreased (-0.06%) to 81.051% when pulling 1d451b8 on uni into a39e7ba on master.