Add pluto submodule.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Thu Jul 16 04:53:18 2015 -0700
#
# On branch master
# Changes to be committed:
#	new file:   .gitmodules
#	new file:   pluto
#
