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.
But now: happiness! 560ce6a734206bccb58343b248cf0216ccdfd178