Standard format for Caffe models to enable easy distribution with forthcoming "model zoo"

This PR will impose a certain structure on all example models we distribute, and add scripts to

- easily download trained models
- push/pull model definitions and readmes to Gists, such that models can be easily distributed but still versioned.

Consult `docs/model_zoo.md` for the proposed changes.
See above -- let's keep the `caffe_` prefix.