MAINT: re-organize setup.py & make sure all tests are discoverable by nose

sklearn/cluster/setup.py

sklearn/cross_decomposition/tests/__init__.py

sklearn/datasets/setup.py

sklearn/feature_extraction/setup.py

sklearn/manifold/setup.py

sklearn/manifold/tests/__init__.py

sklearn/metrics/setup.py

sklearn/model_selection/tests/__init__.py

sklearn/neighbors/setup.py

sklearn/neural_network/tests/__init__.py

sklearn/setup.py

sklearn/utils/setup.py