MRG: AdaBoost for regression and multi-class classification

This PR adds:

* a new ensemble.weight_boosting module with AdaBoostRegressor (using AdaBoost.R2 [1]) and AdaBoostClassifier (using the multi-class SAMME algorithm [2])
* a new "Gaussian quantiles" dataset in datasets.samples_generator as used in [2]

Examples are provided:

![hastie](https://f.cloud.github.com/assets/202816/74736/07057cd0-60a3-11e2-87e3-dd8572078d11.png)

![twoclass](https://f.cloud.github.com/assets/202816/121925/d7a0c70c-6e11-11e2-8927-e0471292262f.png)

![multiclass](https://f.cloud.github.com/assets/202816/121927/dceefd78-6e11-11e2-8959-60c051ef6118.png)

![regression](https://f.cloud.github.com/assets/202816/121928/e1155136-6e11-11e2-9cf2-1d51cae3969a.png)

[1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.31.314
[2] http://www.stanford.edu/~hastie/Papers/samme.pdf