Add __all__ for half of the scikit

sklearn/cluster/__init__.py

sklearn/covariance/__init__.py

sklearn/cross_validation.py

sklearn/datasets/__init__.py

sklearn/decomposition/__init__.py

sklearn/feature_extraction/__init__.py

sklearn/feature_extraction/image.py

sklearn/feature_extraction/text.py

sklearn/feature_selection/__init__.py

sklearn/gaussian_process/__init__.py

sklearn/grid_search.py

sklearn/hmm.py

sklearn/lda.py

sklearn/semi_supervised/__init__.py