SPI-1199: Hadoop XRootD Connector

The new package hadoop_xrootd is added. It is built with mvn, the Maven packagae manager for Java. As it combines Java and C++ code, the default compiler (from contrib) is used internally as well. The paths to the xrootd headers and libraries must be passed for the build to be successful.

As build artifact a so-called fat JAR file is produced that is copied to lib and referenced during the view creating in setup.sh and setup.csh

Documentation is added in packages.json