Roadmap

we are still working on porting the codes from https://github.com/dmlc/wormhole/tree/master/learn/difacto and also adding new learning algorithms:
- [x] logit / fm loss: `src/loss/`
- [x] minibatch sgd: `src/sgd/`
- [x] block coordinate descent: `src/bcd`
- [x] l-bfgs: `src/lbfgs`
- [ ] distributed by ps-lite: `src/store/store_dist.h`
- [ ] use disk as data cache: `class DataStoreDisk`
- [ ] save model
- [ ] predict
- [ ] user doc
- [ ] developer doc