implement GBRT ensembles

[zenogantner]

Implement ensembles based on gradient-boosted regression trees.

For that, we need a C# implementation of GBRT. The Accord.NET framework would be starting point, as it already contains implementations of ID3 and C4.5: http://crsouza.blogspot.com/2012/01/decision-trees-in-c.html

http://en.wikipedia.org/wiki/Gradient_boosting

[zenogantner]

Will not do this unless there is a specific need.