[R] various R code maintenance (#1964)

* [R] xgb.save must work when handle in nil but raw exists

* [R] print.xgb.Booster should still print other info when handle is nil

* [R] rename internal function xgb.Booster to xgb.Booster.handle to make its intent clear

* [R] rename xgb.Booster.check to xgb.Booster.complete and make it visible; more docs

* [R] storing evaluation_log should depend only on watchlist, not on verbose

* [R] reduce the excessive chattiness of unit tests

* [R] only disable some tests in windows when it's not 64-bit

* [R] clean-up xgb.DMatrix

* [R] test xgb.DMatrix loading from libsvm text file

* [R] store feature_names in xgb.Booster, use them from utility functions

* [R] remove non-functional co-occurence computation from xgb.importance

* [R] verbose=0 is enough without a callback

* [R] added forgotten xgb.Booster.complete.Rd; cran check fixes

* [R] update installation instructions

R-package/R/xgboost.R

@@ -1,4 +1,4 @@
-# Simple interface for training an xgboost model.
+# Simple interface for training an xgboost model that wraps \code{xgb.train}
 # Its documentation is combined with xgb.train.
 #
 #' @rdname xgb.train
@@ -12,11 +12,9 @@ xgboost <- function(data = NULL, label = NULL, missing = NA, weight = NULL,
 
   dtrain <- xgb.get.DMatrix(data, label, missing, weight)
 
-  watchlist <- list()
-  if (verbose > 0)
-    watchlist$train = dtrain
+  watchlist <- list(train = dtrain)
 
-  bst <- xgb.train(params, dtrain, nrounds, watchlist, verbose = verbose, print_every_n=print_every_n,
+  bst <- xgb.train(params, dtrain, nrounds, watchlist, verbose = verbose, print_every_n = print_every_n,
                    early_stopping_rounds = early_stopping_rounds, maximize = maximize,
                    save_period = save_period, save_name = save_name,
                    xgb_model = xgb_model, callbacks = callbacks, ...)