ENH better 'constant' learning rate schedule

I switched the mnist to be deeper and narrower using SGD + momentum with a better "constant" learning rate that makes it possible to be aggressive without diverging:

```
...
Iteration 49, cost = 0.00434729

Classification performance:
===========================

Classifier               train-time   test-time   error-rate
------------------------------------------------------------
MultilayerPerceptron        157.24s       0.08s       0.0199
```