API: Change 'clustering' to 'cluster'

examples/cluster/README.txt

examples/cluster/plot_affinity_propagation.py

examples/cluster/plot_lena_segmentation.py

examples/cluster/plot_mean_shift.py

examples/cluster/plot_segmentation_toy.py

examples/stock_market.py

scikits/learn/cluster/__init__.py

scikits/learn/cluster/affinity_propagation_.py

scikits/learn/cluster/k_means_.py

scikits/learn/cluster/mean_shift_.py

scikits/learn/cluster/spectral.py

scikits/learn/cluster/tests/__init__.py

scikits/learn/cluster/tests/common.py

scikits/learn/cluster/tests/test_affinity_propagation.py

scikits/learn/cluster/tests/test_k_means.py

scikits/learn/cluster/tests/test_mean_shift.py

scikits/learn/cluster/tests/test_spectral.py