Introduce Pytest, Tox, Coverage

Fixes #13.

**Still work in progress.** Qemu on Travis is very slow :/

The question is whether we actually need Qemu with a real server, or if we can go with mocking one. Then we could get rid of the Qemu setup.
No :( (though I can't find the link I normally wave at people when this comes up - @ghickman got one handy?)

TL;DR `__init__` woe, filepath woe, general woe, sadness