Clean up of libhdfs src management.

Simplification in the libhdfs src directory structure and relative
changes in setup.py
Fixed #24.

It was an upstream error in hdfs.c (hdfsListDirectory), errno is left in
an undefined state if everything goes well. This was not a problem until
we decided to be very thorough error checkers.
Renamed libhdfs/hadoop-x dirs to libhdfs/x
Fixed #24 for all the currently supported hadoop versions.