Multi-Layer Perceptron

This is a continuation of the GSoC project by David Marek and @amueller. I tried accelerating the implementation using manual calls to CBLAS. 

The performance increase (5%/7% on digits dataset for minibatch=100/online) comes at the price of some readability. Aside from multiplications, the softmax/multinomial logistic loss computation takes a lot of time. However, unning it in C using code from @larsmans mlperceptron branch does not increase efficiency significantly, though.

I'd be happy for suggestions on how to proceed further.