COSMIT + MISC pep8, pyflakes, typos and some other cleanup of examples.

examples/cluster/plot_cluster_comparison.py

examples/cluster/plot_cluster_iris.py

examples/cluster/plot_digits_agglomeration.py

examples/cluster/plot_lena_compress.py

examples/decomposition/plot_pca_3d.py

examples/decomposition/plot_pca_iris.py

examples/exercises/plot_cv_digits.py

examples/exercises/plot_digits_classification_exercise.py

examples/linear_model/plot_iris_logistic.py

examples/linear_model/plot_ols.py

examples/linear_model/plot_ols_3d.py

examples/linear_model/plot_ols_ridge_variance.py

examples/manifold/plot_compare_methods.py

examples/manifold/plot_swissroll.py

examples/mixture/plot_gmm_classifier.py

examples/mixture/plot_gmm_sin.py

examples/plot_digits_last_image.py

examples/plot_digits_pipe.py

examples/plot_hmm_sampling.py

examples/plot_iris_classifiers.py