Move Event.data into abstract node storage

This allows us to split off the (very heavy) storage costs of Event.data and rely on SQL for what it's good at: indexing schemas.

- A new NodeField is included which handles the transitionary phase automatically.
- Event.data is now using NodeField
- SENTRY_NODESTORE options are added, which default to the Django backend (a new simple key/value table)
- A Riak backend is available, and will be the recommended storage backend for large scale deployments.