[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/man/xgb.Booster.complete.Rd

@@ -28,8 +28,8 @@ E.g., when an \code{xgb.Booster} model is saved as an R object and then is loade
 its handle (pointer) to an internal xgboost model would be invalid. The majority of xgboost methods 
 should still work for such a model object since those methods would be using 
 \code{xgb.Booster.complete} internally. However, one might find it to be more efficient to call the  
-\code{xgb.Booster.complete} function once after loading a model as an R-object. That which would
-prevent further reconstruction (potentially, multiple times) of an internal booster model.
+\code{xgb.Booster.complete} function explicitely once after loading a model as an R-object.
+That would prevent further repeated implicit reconstruction of an internal booster model.
 }
 \examples{
 
@@ -41,6 +41,7 @@ saveRDS(bst, "xgb.model.rds")
 bst1 <- readRDS("xgb.model.rds")
 # the handle is invalid:
 print(bst1$handle)
+
 bst1 <- xgb.Booster.complete(bst1)
 # now the handle points to a valid internal booster model:
 print(bst1$handle)