ENH: added PresortBestSplitter
ENH: remove assert
COMIST: shorten some long lines
DOC: added entry to What's New
Merge pull request #2469 from glouppe/tree-presort

[MRG] Make GBRT faster by presorting X
TEST: add tests for splitter="presort-best"
Merge pull request #2474 from glouppe/tree-tests

[MRG] Add tests for splitter="presort-best"
fix broken logo links on homepage and remove unused duplicate images