Generate thumbnails in the example gallery

doc/logos/scikit-learn-logo-thumb.png

doc/sphinxext/gen_rst.py

examples/covariance/plot_covariance_estimation.py

examples/manifold/plot_lle_digits.py.prof

examples/plot_precision_recall.py

examples/plot_roc.py

examples/plot_train_error_vs_test_error.py