libhdfs++: for consistency, include files should be in hdfspp

The existing hdfs library resides in hdfs/hdfs.h. To maintain Least Astonishment, we should move the libhdfspp files into hdfspp/hdfspp.h (they're currently in the libhdfspp/ directory).

Likewise, the install step in the root directory should put the include files in /include/hdfspp and include/hdfs (it currently erroneously puts the hdfs file into libhdfs/)