[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_dmatrix.R

@@ -21,6 +21,15 @@ test_that("xgb.DMatrix: basic construction, saving, loading", {
   dtest3 <- xgb.DMatrix(tmp_file)
   unlink(tmp_file)
   expect_equal(getinfo(dtest1, 'label'), getinfo(dtest3, 'label'))
+  
+  # from a libsvm text file
+  tmp <- c("0 1:1 2:1","1 3:1","0 1:1")
+  tmp_file <- 'tmp.libsvm'
+  writeLines(tmp, tmp_file)
+  dtest4 <- xgb.DMatrix(tmp_file)
+  expect_equal(dim(dtest4), c(3, 4))
+  expect_equal(getinfo(dtest4, 'label'), c(0,1,0))
+  unlink(tmp_file)
 })
 
 test_that("xgb.DMatrix: getinfo & setinfo", {