A java wrapper of XGBoost in the maven central repository

[maropu]

Since there is currently no xgboost library in the maven central repo., we have bundled it in the hivemall package (https://github.com/myui/hivemall/tree/master/xgboost/lib). This is a workaround (I followed this direction) and so this should be resolved until a next release as Apache.

One of solutions is to spin-off only a java wrapper in the xgboost repository; to publish it into the maven repo., we simplify codes for maintainability and add multiple builtin binaries for heterogeneous platforms like linux/x86_64 and mac/x86_64 (use snappy-java as a reference for portability).