Revert "Revert "Add flag to activate error backprop.""

This reverts commit 929040693789f4f02363941341df41bcc673c632.
Do not backpropagate in first layer.
Add normalization.
Add python binding for normalization.
Use normalization from OpenANN.
Fix chaining.
Add PCA.
Add Python bindings.
Set seed.
Matrix U is not required.
Add a sentence that describes the PCA.
Merge pull request #126 from AlexanderFabisch/pca

Pca