[R] more attribute handling functionality

R-package/NAMESPACE

@@ -10,6 +10,8 @@ S3method(predict,xgb.Booster.handle)
 S3method(setinfo,xgb.DMatrix)
 S3method(slice,xgb.DMatrix)
 export("xgb.attr<-")
+export("xgb.attributes<-")
+export("xgb.parameters<-")
 export(getinfo)
 export(print.xgb.DMatrix)
 export(setinfo)
@@ -17,6 +19,7 @@ export(slice)
 export(xgb.DMatrix)
 export(xgb.DMatrix.save)
 export(xgb.attr)
+export(xgb.attributes)
 export(xgb.create.features)
 export(xgb.cv)
 export(xgb.dump)