Bash tab completion support for the hadoop script.

On Debian-like distributions, the script can be placed in
/etc/bash_completion.d/, and it will be sourced automatically by Bash. On
other distributions, you may source the file manually (`. hadoop.sh') or
source it from your bashrc (or equivalent) file.

The script allows tab completion of all the command names, subcommands for the
'fs', 'dfsadmin', 'job', 'namenode' and 'pipe' commands, arguments of the 'jar'
command and most arguments to the 'fs' subcommands (completing local and 
dfs paths as appropriate).