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
PEP8
Merge pull request #1712 from glouppe/tree-maxfeatures

[MRG] Support float values for max_features