[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/xgboost.R

@@ -1,4 +1,4 @@
-# Simple interface for training an xgboost model that wraps \code{xgb.train}
+# Simple interface for training an xgboost model that wraps \code{xgb.train}.
 # Its documentation is combined with xgb.train.
 #
 #' @rdname xgb.train
@@ -7,7 +7,7 @@ xgboost <- function(data = NULL, label = NULL, missing = NA, weight = NULL,
                     params = list(), nrounds,
                     verbose = 1, print_every_n = 1L, 
                     early_stopping_rounds = NULL, maximize = NULL, 
-                    save_period = 0, save_name = "xgboost.model",
+                    save_period = NULL, save_name = "xgboost.model",
                     xgb_model = NULL, callbacks = list(), ...) {
 
   dtrain <- xgb.get.DMatrix(data, label, missing, weight)