ENH: Refactor the datasets to add the digits standard dataset.

examples/lda.py

examples/naive_bayes.py

examples/plot_feature_selection.py

examples/plot_neighbors.py

examples/plot_svm.py

scikits/learn/datasets/__init__.py

scikits/learn/datasets/base.py

scikits/learn/datasets/data/digits.csv.gz

scikits/learn/datasets/descr/digits.rst

scikits/learn/datasets/descr/iris.rst