moved sgd examples to examples/linear_model and added sgd prefix.

benchmarks/bench_sgd_covertype.py

doc/modules/sgd.rst

examples/linear_model/plot_sgd_iris.py

examples/linear_model/plot_sgd_loss_functions.py

examples/linear_model/plot_sgd_ols.py

examples/linear_model/plot_sgd_penalties.py

examples/linear_model/plot_sgd_separating_hyperplane.py

examples/linear_model/plot_sgd_weighted_classes.py

examples/sgd/README.txt