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.
I'm pretty sure examples should still be ignored. If you execute any ipynb it will change the contents since new matplotlib figures are generated but you hardly want to commit that.