Simple maxout brick

Okay, probably adding configuration to activations and losing their statelessness is too high cost for implementing Maxout. I created a pull request with more localized changes: 

https://github.com/bartvm/blocks/pull/50

The idea is that all the activations either keep the input dimension or reduce it k times. @pbrakel , @bartvm , please tell me what you think about such change in the MLP framework.