Download learning-to-rank JAR file with all dependencies

This is a copy of the original RankLib from the Lemur project (https://sourceforge.net/p/lemur/wiki/RankLib/). The copy serves as a possibility to create a Maven artifact uploaded to Maven central since the Lemur project doesn't provide Maven artifacts on central. Additionally, changes have been made to use RankLib more easily as a library from the code of another program. Also, for thread-safety, the static, global feature number counter is not used any more.