adding fully Spatial network (for doing detection)

This PR is not meant to be merged off the bat but if someone is interested, this gives:
- detection mode (so that you can send in larger images and get dense responses for each location)
- just saves the model to disk using torch-serialization, so that you can reuse it as a regular model everywhere
https://github.com/soumith/overfeat-torch/blob/master/run.lua#L89

- Implemented using CuDNN (for SpatialSoftMax), so GPU-only