setup gem user stamps

Changes in this PR:

1. setup gem userstamp and models

2. userstamp is not compatible with existing gem schema_plus, because the column name is creator_id and updater_id, which schema_plus will add foreign keys to them. Fixed by set userstamp to compatible mode.
REF: https://github.com/lowjoel/userstamp/blob/master/lib/stampable.rb#L12
This fixes the symptoms but doesn't solve the problem. I suggest you override Userstamp to generate the proper foreign keys.