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.
The default `open` is not a good idea because it does not always handle encoding correctly, see https://github.com/strongswan/swidGenerator/pull/35. If the system has a terminal which is not set to UTF-8, it could cause problems.

Using a context manager is a good idea though.