Use tox to run tests

[legoktm]

tox makes testing extremely easy, see https://tox.readthedocs.org/en/latest/
for more details.

Converted .travis.yml to use it.

[Julian]

tox doesn't add much when you're not testing multiple versions, and comes with its own set of pitfalls. It also doesn't interact well with travis (mostly only when you're testing multiple versions).

I'm not against this change, but I'm "meh" about it. Letting it stew for a tiny bit to see how I feel in the morning :)?

[legoktm]

I mainly find tox useful because it creates a standardized way to run tests. I don't need to remember the specific command to run tests for whatever project I'm working on, I can just run tox and know that the appropriate tests are being run.

[Julian]

I tend to much prefer that testrunner package is that uniform interface, but since this isn't a normal package obviously, it might make sense here I guess.

[Julian]

Sorry that this sat here but I guess I'm OK with it, so merged :) Thanks.