Added SimpleSGD item recommender

Hey Zeno. 
I've been working on this algorithm. It's been published at UMAP this year (2014).
It's a really simple approach. It still has some problems, i'm dealing with right now, so i'll be contributing with more related to this.

PS: I don't really get most of the changes in the project file, maybe it's some kind of architecture issue...
 
Cheers,
João Vinagre