Group JarCreate jarring actions in a jar-create MULTITOOL workunit.

Testing Done:
Before:
=======
$ PANTS_DEV=1 ./pants goal bundle --bundle-deployjar src/java/com/pants/examples/pingpong/::
...
08:04:20 00:08   [jar]
08:04:20 00:08     [jar]
08:04:20 00:08       [bootstrap-jar_tool]
08:04:22 00:10       [jar-tool]
08:04:22 00:10       [jar-tool]
08:04:22 00:10   [bundle]
08:04:22 00:10     [bundle]
                   creating dist/pingpong-bundle
...

After (note the dot per collapsed jar-tool workunit):
======
$ PANTS_DEV=1 ./pants goal bundle --bundle-deployjar src/java/com/pants/examples/pingpong/::
...
08:04:38 00:01   [jar]
08:04:38 00:01     [jar]
08:04:38 00:01       [jar-create]..
08:04:38 00:01   [bundle]
08:04:38 00:01     [bundle]
                   creating dist/pingpong-bundle

Bugs closed: 164

Reviewed at https://rbcommons.com/s/twitter/r/416/