Document Caffe by a tour of key subjects

Lead a tour through the main points of how Caffe works and how to work with Caffe.

The Story
- [x] nets, layers, and blobs
- [x] forward / backward passes
- [ ] solving
- [x] loss
- [x] interfaces: cli, python, matlab
- [ ] figures

Details
- [x] layer catalogue skeleton
- [ ] data, IO
- later: convolution
- later: add Net::Init to nets, layers, blobs
- later: DAGs
- later: weight sharing

Comments
- [x] Doxygen generation
- [ ] add brief + TODO stubs everywhere
- later: comment crusade

@Yangqing @jeffdonahue let's collaborate on my fork to write this all up and edit this description as you like. BVLC folk, feel free to help!