FIX error message from trees for large inputs

Fixes #2809 (hopefully).
DOC: minor improvement in comments of an example
Revert "ENH sparsify and densify methods for CD models"

This reverts commit ee85da8421a35894620e86dbd5b0013b5628812f;
shouldn't have been committed to master.
Merge pull request #2864 from sciunto/doc

DOC: minor improvement in comments of an example
MAINT missing import_array() in isotonic r. Cython code

Not extremely important, but issues one less compiler warning.
MAINT get rid of compiler warnings from Liblinear
COSMIT: many cosmetic comments

Fixes remarks by @ogrisel, @agramfort, @larsman, @jnothman
FIX: fix failing doctest
DOC: link FeatureAgglomeration better to dim red
MISC: documentation and cosmits

Address comments by @jnothman
DOC no more "arithmetical order" for classes
BF: load_boston() return 13 features, but 14 feature names

Last column has the target value, hence only report first 13 names.
BUG: work aroundd Cython bug to build with clang

This commit should work around a Cython bug that makes the previous
iteration code in fast_dict.pyx impossible to compile under clang. See
https://mail.python.org/pipermail/cython-devel/2013-April/003566.html
Fixes Issue 2751
Merge pull request #2865 from hanke/boston_13features

BF: load_boston() returns 13 features, but 14 feature names
Merge branch 'master' into hc_linkage

Conflicts:
	doc/whats_new.rst
ENH Cache features value for extra trees
FIX: Removed coef and improved initialization
Merge pull request #2866 from Manoj-Kumar-S/Iss2751

Fixes Issue 2751
FIX integer dtype for labels in DBSCAN

Fixes #2833.