Bug 976223 - Add service-related data persistence layer r=abr

This patch implements two distinct persistence layers sharing
the very same API for storing services-related data:

- `MongoStore` using MongoDB as a backend
- `MemoryStore` storing data in-memory
Merge pull request #10 from mozilla/simple-keyvalue-store

Bug 976223 - Add service-related data persistence layer. r=abr