Added future item support

- Add new commands to add prediction models and assigning models to items.
- Semantics of `sync-db` has changed: now it only syncs history for items present in item_future table, and also executes the predictions (see below).
- Predictions are expected to be executables in `$PWD/future_models/<name>', (ie. future_model.name = <name>), that are fed Events in standard input and are expected to output Result to stdandard output. See multi-axis-graphs wiki for documentation on these JSON objects.