Merge branch 'sgd'

doc/modules/sgd.rst

examples/sgd/README.txt

examples/sgd/plot_loss_functions.py

examples/sgd/plot_penalties.py

examples/sgd/plot_separating_hyperplane.py

scikits/learn/sgd/base.py

scikits/learn/sgd/sparse/__init__.py

scikits/learn/sgd/sparse/sgd.py

scikits/learn/sgd/sparse/src/sgd_fast_sparse.c

scikits/learn/sgd/sparse/src/sgd_fast_sparse.pyx

scikits/learn/sgd/tests/test_sparse.py