Publish entity changes to amazon sns

The PR implements Pub/Sub functionality using Amazon Simple Notification Service.

Root entity types are assigned a topic whose name is derived from the entity type's name.  (E.g., a type named 'person' will be assigned the topic 'gr_person')

Deployment notes
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Before deployment: 
1. an aws.yml file with an appropriate amazon key must be created in the config dir on each server 
2. the db migration must be performed.  
3. the sns:create_topics rake task must be performed

Summer Project app should at some point treat summer_project_application as a root entity.  Immediately before that change is deployed, "rake gr:make_root_type[3]" should be run. (3 is the id of the summer_project_application entity_type.)

http://jira.uscm.org/browse/GR-1