SCHEMA-175. kiji shell script should be able to run kiji-mapreduce tools...

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This commit modifies the kiji script so that if KIJI_MR_HOME is set, jars in
$KIJI_MR_HOME/lib are added to the classpath of the kiji script. This allows
KijiToolLauncher to launch tools provided in kiji-mapreduce. Help messages are
updated to notify the user that they should set this environment variable if
they wish to use kiji-mapreduce tools.