Twenty Newsgroups Classification Example fails in case running with MAHOUT_LOCAL=true

Twenty Newsgroups Classification Example fails in case running with MAHOUT_LOCAL=true or else when HADOOP_HOME env variable is not set.

Newsgroups lists instructions in order to run this classifier. When running in standalone mode(MAHOUT_LOCAL=true), i.e., running $ ./examples/bin/classify-20newsgroups.sh, the script runs ./examples/bin/set-dfs-commands.sh internally to export hadoop related env variables.

set-dfs-commands.sh attempts to check for hadoop version despite running with MAHOUT_LOCAL set as true. IMHO, the script works fine considering the prerequisites, but, it will as well make sense if we can update the script ./examples/bin/set-dfs-commands.sh to export hadoop env varibales only in case MAHOUT_LOCAL is not set to true.