Travis-CI integration

Note that, in order to make brainx work under Python 3, I had to change a lot of ``itervalues`` to ``values``.  This means that the whole list will be returned under Python 2.7 now, which might slow down some very large networks.  If this poses a practical problem, we can use the ``six`` compatibility module to get back the old behavior under 2.7 using ``six.itervalues``.