DOC make neural networks example appear

Also make the absence of a README fail doc compilation
DOC add links to github sourcecode in API reference
Merge pull request #3460 from jnothman/fix_nn_example_doc

[MRG+2] DOC make neural networks example appear
Merge pull request #2777 from jnothman/doc_linkcode

[MRG+2] DOC add links to github sourcecode in API reference
Update outlier_detection.rst

Add reference for One-class SVM.
DOC fix opaque background glitch when hovering example icons
FIX better RandomizedPCA sparse deprecation
Merge pull request #3470 from ogrisel/fix-sparse-randomized-pca-deprecation

[MRG] better RandomizedPCA sparse deprecation
DOC: Fix class link

Typos, typos, typos :)
MAINT move log_loss and hinge_loss to the classification metrics
Merge pull request #3472 from arjoly/fix-metrics-division

MAINT move log_loss and hinge_loss to the classification metrics
ENH: Added n_iter_ parameters across all iterative solvers
FIX: Preserve public API by introducing return_n_iter param
TST: Test that after fitting n_iter is greater than 1
Added n_iter attribute to OMP (and CV)
MAINT: Fixed a few docstrings and cosmits
MAINT remove deprecated PCA code
MAINT remove deprecated code from trees and forests
MAINT remove deprecated sklearn.pls module
MAINT remove deprecated code from preprocessing