add transaction test
Merge pull request #172 from manuzhang/add_transaction_test

add unit test to transaction package and fix bugs
add grouper interface and locality aware grouper
Merge pull request #173 from manuzhang/add_locality_aware_grouper

Add grouper interface and locality aware grouper
update hierarchy picture and readme
update readme
Merge branch 'manuzhang-update_doc_picture' into upstream_master
Fixes #176
Merge pull request #177 from kkasravi/issue_176

Fixes #176