* [gblinear] add features contribution prediction; fix DumpModel bug * [gbtree] minor changes to PredContrib * [R] add feature contribution prediction to R * [R] bump up version; update NEWS * [gblinear] fix the base_margin issue; fixes #1969 * [R] list of matrices as output of multiclass feature contributions * [gblinear] make order of DumpModel coefficients consistent: group index changes the fastest