requisition notifications 

sorry this commit is so big. nothing ever really got unglued enough to be
able to break it up into smaller chunks. here's the summary:
- send notifications to interested parties when requisitions are updated
  so that they can take the next step in the workflow
- this is done by finding the requisition case owner and looking for
  a user with the appropriate role set for the next requisition state
- a side effect was adding owner inheritance between supply points,
  supply point products, and requisitions
- added a bunch of constants and utility functions to make things cleaner
- had to update the tests quite a bit to generate an appropriate set of
  people to notify on requisition changes
- i verified the tests by looking at the output of the sms log to console,
  still need to write something that can do better sms scripting unit
  testing a-la rapidsms