decision_function forces dense output (in the case of sparse coef_)
FIX: use DOUBLE_t type
Merge pull request #1705 from glouppe/tree-fix

[MRG]: FIX: use DOUBLE_t type
COSMIT pep8: blank line contains whitespace
DOC added sentence about oob_decision_function_ containing NaN to docstring. Still need some narrative about oob score.
ENH sparsify method for L1-reg linear models

Currently implemented on LinearSVC, LR and SGD.
DOC developer guidelines for unit tests and classes_
DOC add 0.13.1 changelog to whats_new.rst
DOC dev guide: random_state_ + @amueller's remarks

Fixes #1707.
DOC r2_score may return negative values
DOC dev guide: deprecation
ENH: support float value for max_features
DOC: if float, then max_features is a percentage
ENH: Defer parameter checking of trees
DOC: GBRT max_features
TEST: added test
ENH: use numbers
FIX: numpy integers
Merge branch 'sparse-coef'

Conflicts:
	doc/whats_new.rst
COSMIT callable instead of hasattr __call__