Flakes for CI

* I've switched on flakes in CI so that it can catch some simple errors.
* Among cruft pyflakes has identified two actual bugs.

Right now pyflakes is too simple-minded to digest dynamic formation of `__all__` in `__init__.py`, should we do something about it or should we rather silence pyflakes like I did?

See merge request !29

switch to py.test

Closes #37.

Thanks to @jbweston for setting up a proper CI image.

See merge request !21

The date is currently missing, and should be provided before
an official release is made.

Also, simplify site family comparison test.

Specifying this is presently optional, in order to remain
backwards compatibility, but will be compulsory in Kwant2.

The inverse map is created during finalization anyway, now
we just save it along with the `sites` map.

fixes #36

this fixes #35