Remove duplicate GaussianNB.fit() code

This code is substantially duplicated by partial_fit(), so just call
partial_fit().
Preserve public API
docstring
Merge pull request #3344 from ihaque/remove_dup_fit

Remove duplicate GaussianNB.fit() code
DOC double backticks for fixed-width (code) font

Fixes #3337. Also other minor fixes while I was at it.
TST crank up preprocessing tests to 99% coverage

Fixes #3347. (nosetests reports no missing lines, so 99% seems to
be a roundoff error, or a hint that 100% coverage is impossible :p)
FIX Gaussian KDE should return array, not scalar

Fixes #3269.
COSMIT use explicit RandomState in KDE tests
ENH make_multilabel_classification for large n_features: faster and sparse output support
COSMIT in response to @arjoly's comments
Update what's new
FIX failing tests due to the change of the underlying sample generator
DOC more explicit docstring for preprocessing.normalize
Added utility to skip tests if running on Travis
DOC optimize some PNG images, shaves off 24kB
DOC run optipng before uploading website
Merge pull request #3355 from larsmans/optipng

[MRG+1] DOC run optipng before uploading website
Fix grammar

it's should be its
Merge pull request #3357 from houbysoft/patch-1

Fix grammar
Fix labels of inliers vs outliers

At least based on what svm.OneClassSVM does in the example linked from this document, the description was wrong; labels actually assigned are 1/-1 for good/bad respectively, not 0/1.