DOC allow link embedding to proceed when web sites unavailable
DOC explain SelectorMixin strategy with multiple classes.
DOC Add note about sample weights in export_graphviz.
Merge pull request #3829 from amueller/feature_selection_from_model_doc

DOC explain SelectorMixin strategy with multiple classes.
FIX raise error when number of features changes in partial fit, Add common test for inconsisten number of features for partial fit.
Fixed issue 3815. Discrete AdaBoostClassifier now fails early if the base classifier if worse than random.
unused variables are redefined later
FeatureUnion example using a heterogeneous datasource.

Uses the 20newsgroups dataset. Extracts & processes the subject-line and body
with separate pipelines and combines them with a FeatureUnion.
DOC RidgeCV more explicit documentation of CV parameter, taken from GridSearchCV.
Merge pull request #3571 from jnothman/fix-link-embedding

[MRG] DOC allow link embedding when some sites not available
MAINT CI: collect rackspace DNS info

Sometimes the following installation step fails:

    pip install -r continuous_integration/appveyor/requirements.txt

This steps download prebuilt version of numpy and scipy for windows that
are hosted on a rackspace container. In order to be able to report
the necessary debugging info to the rackspace cloud support team we need
to know the location of the failing hosts.
MAINT try to avoid HTTP timeouts to rackspace
Update about page
Remove unused distance calculation from clustering example.
DOC link to NLTK website

[ci skip]
FIX #2986: ZeroDivisionError in LinearRegression on sparse data
DOC: add funding from Paris-Saclay Centre for Data Science

Also:
* Completed INRIA funding description
* Fixed Google Summer Of Code rst link
DOC FIX: typo and minor update
Merge pull request #3854 from justmarkham/doc-changes

DOC FIX: typo and minor update
Merge pull request #3833 from amueller/sgd_partial_fit_variable_length

[MRG] TST raise error when number of features changes in partial fit