[MRG] DOC Prediction Speed

As discussed with Olivier and Lars on the list, here is a PR to incorporate documentation and examples related to prediction latency.

Here is the list of tasks:

- [x] create topical / howto section on performance (not prediction power)
- [x] cover atomic / bulk prediction mode
- [x] cover influence of n_features
- [x] cover influence of input data representation
- [x] cover prediction throughput
- [x] cover feature extraction
- [x] cover blas/lapack libraries
- [x] cover model compression (aka sparsify)