[GPU-Plugin] Integration of a faster version of grow_gpu plugin into mainstream (#2360)

* Integrating a faster version of grow_gpu plugin
1. Removed the older files to reduce duplication
2. Moved all of the grow_gpu files under 'exact' folder
3. All of them are inside 'exact' namespace to avoid any conflicts
4. Fixed a bug in benchmark.py while running only 'grow_gpu' plugin
5. Added cub and googletest submodules to ease integration and unit-testing
6. Updates to CMakeLists.txt to directly build cuda objects into libxgboost

* Added support for building gpu plugins through make flow
1. updated makefile and config.mk to add right targets
2. added unit-tests for gpu exact plugin code

* 1. Added support for building gpu plugin using 'make' flow as well
2. Updated instructions for building and testing gpu plugin

* Fix travis-ci errors for PR#2360
1. lint errors on unit-tests
2. removed googletest, instead depended upon dmlc-core provide gtest cache

* Some more fixes to travis-ci lint failures PR#2360

* Added Rory's copyrights to the files containing code from both.

* updated copyright statement as per Rory's request

* moved the static datasets into a script to generate them at runtime

* 1. memory usage print when silent=0
2. tests/ and test/ folder organization
3. removal of the dependency of googletest for just building xgboost
4. coding style updates for .cuh as well

* Fixes for compilation warnings

* add cuda object files as well when JVM_BINDINGS=ON

plugin/updater_gpu/benchmark/benchmark.py

@@ -42,7 +42,7 @@ args = parser.parse_args()
 
 if 'grow_gpu_hist' in args.algorithm:
     run_benchmark(args, args.algorithm, 'grow_fast_histmaker')
-if 'grow_gpu ' in args.algorithm:
+if 'grow_gpu' in args.algorithm:
     run_benchmark(args, args.algorithm, 'grow_colmaker')
 if 'all' in args.algorithm:
     run_benchmark(args, 'grow_gpu', 'grow_colmaker')