visualization of the conv layer kernels

This is basically an adopted version of the conv layer kernel visualization from: https://code.google.com/p/cuda-convnet/source/browse/trunk/shownet.py

Here's for example the first layer of the imagenet model, that's available for download:
![out](https://f.cloud.github.com/assets/363737/2257133/cb31e374-9e13-11e3-8947-74a6f95f52ff.png)