Add sink for Prometheus monitoring system (#46)

* Add sink for Prometheus monitoring system

* Add prometheus sink to README

sinks/prometheusSink.md

@@ -0,0 +1,23 @@
+## `prometheus` sink
+
+The `prometheus` sink publishes all metrics via an HTTP server ready to be scraped by a [Prometheus](https://prometheus.io) server. It creates gauge metrics for all node metrics and gauge vectors for all metrics with a subtype like 'device', 'cpu' or 'socket'. 
+
+
+### Configuration structure
+
+```json
+{
+  "<name>": {
+    "type": "prometheus",
+    "host": "localhost",
+    "port": "8080",
+    "path": "metrics"
+  }
+}
+```
+
+- `type`: makes the sink an `prometheus` sink
+- `host`: The HTTP server gets bound to that IP/hostname
+- `port`: Portnumber (as string) for the HTTP server
+- `path`: Path where the metrics should be servered. The metrics will be published at `host`:`port`/`path`
+- `group_as_namespace`: Most metrics contain a group as meta information like 'memory', 'load'. With this the metric names are extended to `group`_`name` if possible.