Add Grafana Alerting support

[joente]

Most likely it is not (only) this datasource but also Grafana itself where changes are required to support alerting for SiriDB.

http://docs.grafana.org/alerting/rules/

The alert rules are evaluated in the Grafana backend in a scheduler and query execution engine that is part of core Grafana. Only some data sources are supported right now. They include Graphite, Prometheus, InfluxDB, OpenTSDB, MySQL, Postgres and Cloudwatch.