[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/tests/testthat/test_monotone.R

@@ -10,8 +10,8 @@ train = matrix(x, ncol = 1)
 
 test_that("monotone constraints for regression", {
   bst = xgboost(data = train, label = y, max_depth = 2,
-                 eta = 0.1, nthread = 2, nrounds = 100,
-                 monotone_constraints = -1)
+                eta = 0.1, nthread = 2, nrounds = 100, verbose = 0,
+                monotone_constraints = -1)
   
   pred = predict(bst, train)