Add gatherer app that handle process requests to founds

[rotem-benzvi]

The commit:

1. gatherer_task is the periodic func that run every hour and handle the process from requests to founds and call the Agora process.
2. Agora.py operate the http request and find the items.
3. for run the gatherer background task from startup we override AppConfig.ready() and use post_migrate in apps.py

fixes: #62, #63, #64, #72 fixed: #56