BENTO-9. Adds kiji-mapreduce and kiji-mapreduce lib to kiji-bento.

This commit modifies the distribution of kiji-bento to include the
latest versions of kiji-mapreduce and kiji-mapreduce-lib. The
script kiji-env.sh is also modified to set KIJI_MR_HOME (which
integrates kiji-mapreduce tools with the kiji script) and the README
file is updated.

Signed-off-by: Aaron Kimball <aaron@wibidata.com>