Implement database backed store

[tim-schilling]

This work should occur off of the serializable branch.

The goal here is to implement a DatabaseStore that inherits from toolbar.store.BaseStore which is similar to MemoryStore.

Some considerations:

• We should be able to run the test suite with the DatabaseStore easily
• When running make example, we should be able to kill the process, start it up and see requests from the first make example run. This proves we're able to fetch data from the database.
• We should clear the data that's older than X hours or days in our AppConfig.ready(). We don't need to retain data from forever. This should be exposed as a setting and documented.

Sub-issues should be created from this to track functonality

[tim-schilling]

From Djangonaut Space meeting:

• Reasonable model design is:
  • uuid key
  • datetime created timestamp,
  • json blob for data

[dr-rompecabezas]

Team Neptune: I'll stop fearing the 'async' label and start working on this. :)