Merge branch 'pr/3247'
Refactoring, small improvements, and cleaning notation of IsotonicRegression
Merge pull request #3240 from JelleZijlstra/fix-defaultdict

FIX defaultdict call for some Python versions
DOC more complete coverage of API linking in examples

Previously ignored non-plot files.

Avoids some link over/undergeneration
TST add test for name identification in RST generation
TST avoid nose running setup() in gen_rst.py
TST add sphinxext to testing
Changing docs from "vector" to "1d array"
FIX use NPY_INFINITY instead of C99 INFINITY for MSVC
Merge pull request #3251 from ogrisel/fix-msvc-INFINITY

[MRG] FIX use NPY_INFINITY instead of C99 INFINITY for MSVC
ENH/DOC fix poly features complexity

Fixes #3191, #3194.
ENH interaction_only in PolynomialFeatures
Merge pull request #3239 from larsmans/faster-poly-features

compute poly features directly and allow interaction features only
COSMIT remove duplicate key 'hamming' in METRIC_MAPPING
FIX PCA error handling for invalid n_components

n_components < 0 was silently handled like None; > n_features gave a
cryptic error message from the bowels of np.linalg.

XXX n_components == 0 is still allowed as changing it makes tests fail,
not sure what it means.
Fix Machine Learning for NeuroImaging in Python link.
Merge pull request #3254 from laurentluce/presentations-links-fix

Fix "Machine Learning for NeuroImaging in Python" link.
TST remove dependencies for testing sphinxext
DOC: fix 2 covariance examples rst math markup.

Some formulas in plot_mahalanobis_distances and
plot_robust_vs_empirical_covariance were not correctly rendered by sphinx
as some characters were not properly escaped. This is fixed by setting the
docstrings as raw string using r"""

There was also missing braces for the number of features and samples subscript.
Branching 0.15.X to prepare the 0.15.0b1 release