Bug in calculating average scores

Scores based on `np.average` are wrong when input is a dictionary. The reason is that `len` is called on the batch, which returns the number of keys on a dict.

See lines [here](https://github.com/dnouri/nolearn/blob/master/nolearn/lasagne/base.py#L738) and [here](https://github.com/dnouri/nolearn/blob/master/nolearn/lasagne/base.py#L747).