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
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
MAINT remove sklearn.test