Trying GradientBoosterClassifier.

Just in case you are interested, I've tinkered a bit with main.py and ensemble.py to try sklearn.GradientBoostClassifier. The results were even worse than what I managed using Weka so I didn't bother making any submission. I've refactored a bit the code according to some of my major comments before. The diff seems to be more dramatic than it is because I've replaced every tab by 4 spaces, so I advise to use "diff -Eb ...".