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.
As long as I just import the meta module, which in turn doesn't import anything from the library, it should be fine, right? The idea was that the code also has access to the program version (e.g. for debug output).

But I can also remove it entirely and just keep the version and other meta-data in the setup.py file :)