Merge pull request #1772 from jnothman/comment_exhaustive_search

DOC Allowing a list of param_grids means GridSearchCV is more than grids
Merge pull request #1773 from jnothman/prf_docstring

DOC clarify relationship between pos_label and average parameters
BUG StandardScaler would ignore with_std for CSR input

Also clarified the docs and added the explanation promised in the
exception message.

Fixes #1724.
DOC add roll your own estimator docs
TYPO fix
FIX for iid weighting in grid-search
DOC FIX finite precision
typo
Clarify docstring in lars_path
BUG SGDClassifier and friends did not forget labels_ in re-fit

Fixes #1703.
ENH optimizations in sklearn.cross_validation
DOC clarify C parameter on LogisticRegression
TST + DOC + COSMIT refactor ParameterGrid and test it
ENH len on ParameterGrid and ParameterSampler
BUG deprecation of grid_scores_ in GridSearchCV

The test still used the old name; the deprecation warning
referred to the wrong attribute name.
BUG always do cross-validation in GridSearchCV

Optimization for special case of one point in the grid caused cross
validation to be skipped entirely. Fixes #1782.
DOC fix clone and get_params documentation
TST grid search/randomized search on non-BaseEstimator

See #1265.
TST actual sparse input in sparse k-NN tests
COSMIT prevent a copy in randomized LR

Also remove some superseded Py3 compat code.