faster/better tree building algorithm (see #64)
faster/better tree building algorithm (see #64)

I haven't validated this with any real dataset, need to do that

subu-cliqz/annoy,subu-cliqz/annoy,LongbinChen/annoy,Houzz/annoy2,pombredanne/annoy,eddelbuettel/annoy,spotify/annoy,eddelbuettel/annoy,subu-cliqz/annoy,spotify/annoy,subu-cliqz/annoy,tjrileywisc/annoy,tjrileywisc/annoy,Houzz/annoy2,guitarmind/annoy,pombredanne/annoy,pombredanne/annoy,codeaudit/annoy,spotify/annoy,tjrileywisc/annoy,eddelbuettel/annoy,pombredanne/annoy,guitarmind/annoy,BeifeiZhou/annoy,guitarmind/annoy,eddelbuettel/annoy,spotify/annoy,Houzz/annoy2,BeifeiZhou/annoy,BeifeiZhou/annoy,LongbinChen/annoy2,LongbinChen/annoy,tjrileywisc/annoy,codeaudit/annoy,LongbinChen/annoy2,LongbinChen/annoy,codeaudit/annoy,LongbinChen/annoy2