Add more tests

[janbrohl]

Checking if the documentation can be built including examples is a test, too - but more rigorous testing should help further development.

[dhellmann]

When I set up this repository, I copied metadata from a project that used testrepository to drive unit tests. As part of this ticket, we should drop all of that and set up py.test instead.

[dhellmann]

See #57 and #58