Events filtering

This pull request implements:

1. /api/v1/sensu/events/filtered, which returns Sensu events for which there is not a corresponding silence stash.
2. Changes routes/monitoring/events.jade to use /api/v1/sensu/events/filtered as the datatables data source.
3. Refactors /api/v1/sensu/events to use app.locals.getSensuEvents() to allow this logic to be shared with /api/v1/sensu/events/filtered