[GPU-Plugin] Add GPU accelerated prediction (#2593)

* [GPU-Plugin] Add GPU accelerated prediction

* Improve allocation message

* Update documentation

* Resolve linker error for predictor

* Add unit tests

tests/cpp/test_learner.cc

@@ -0,0 +1,14 @@
+// Copyright by Contributors
+#include <gtest/gtest.h>
+#include "helpers.h"
+#include "xgboost/learner.h"
+
+namespace xgboost {
+TEST(learner, Test) {
+  typedef std::pair<std::string, std::string> arg;
+  auto args = {arg("tree_method", "exact")};
+  auto mat = {CreateDMatrix(10, 10, 0)};
+  auto learner = std::unique_ptr<Learner>(Learner::Create(mat));
+  learner->Configure(args);
+}
+}  // namespace xgboost