Add LocalOfferManager to get local offers
API change: search APIs

Offer search is now at api/offers/search. A GET at that url with no params
returns every offer in the database. You can specify two parameters now:
- radius: This enables local search. A *mandatory* parameter is ll, the
  lat/long string formatted as $lat,$long
- isbn: Return all offers for the specified isbn.

You can of course specify both parameters.