Coding guidelines

The coding style should mostly follow the Brian 2 guidelines, with one major exception: for brian2tools the code should be both Python 2 (for versions >= 2.7) and Python 3 compatible. This means for example to use range and not xrange for iteration or conversely use list(range) instead of just range when a list is required. For now, this works without from __future__ imports or helper modules like six but the details of this will be fixed when the need arises.