[R] maintenance Apr 2017 (#2237)

* [R] make sure things work for a single split model; fixes #2191

* [R] add option use_int_id to xgb.model.dt.tree

* [R] add example of exporting tree plot to a file

* [R] set save_period = NULL as default in xgboost() to be the same as in xgb.train; fixes #2182

* [R] it's a good practice after CRAN releases to bump up package version in dev

* [R] allow xgb.DMatrix construction from integer dense matrices

* [R] xgb.DMatrix: silent parameter; improve documentation

* [R] xgb.model.dt.tree code style changes

* [R] update NEWS with parameter changes

* [R] code safety & style; handle non-strict matrix and inherited classes of input and model; fixes #2242

* [R] change to x.y.z.p R-package versioning scheme and set version to 0.6.4.3

* [R] add an R package versioning section to the contributors guide

* [R] R-package/README.md: clean up the redundant old installation instructions, link the contributors guide

R-package/R/xgb.plot.importance.R

@@ -61,8 +61,8 @@
 xgb.plot.importance <- function(importance_matrix = NULL, top_n = NULL, measure = NULL, 
                                 rel_to_first = FALSE, left_margin = 10, cex = NULL, plot = TRUE, ...) {
   check.deprecation(...)
-  if (!"data.table" %in% class(importance_matrix))  {
-    stop("importance_matrix: Should be a data.table.")
+  if (!is.data.table(importance_matrix))  {
+    stop("importance_matrix: must be a data.table")
   }
 
   imp_names <- colnames(importance_matrix)
@@ -107,12 +107,12 @@ xgb.plot.importance <- function(importance_matrix = NULL, top_n = NULL, measure
     
     # reverse the order of rows to have the highest ranked at the top
     importance_matrix[nrow(importance_matrix):1,
-                      barplot(Importance, horiz=TRUE, border=NA, cex.names=cex,
-                              names.arg=Feature, las=1, ...)]
+                      barplot(Importance, horiz = TRUE, border = NA, cex.names = cex,
+                              names.arg = Feature, las = 1, ...)]
     grid(NULL, NA)
     # redraw over the grid
     importance_matrix[nrow(importance_matrix):1,
-                      barplot(Importance, horiz=TRUE, border=NA, add=TRUE)]
+                      barplot(Importance, horiz = TRUE, border = NA, add = TRUE)]
     par(op)
   }