Add new httpSink

This sink is compatible with the HTTP API of cc-metric-store.
Example config.json section:

```
"sink": {
    "type": "http",
    "host": "localhost",
    "port": "8080",
    "database": "api/write",
    "password": "<JWT>"
},
```

The password/JWT can be omitted.

metric-collector.go

@@ -36,6 +36,7 @@ var Sinks = map[string]sinks.SinkFuncs{
 	"influxdb": &sinks.InfluxSink{},
 	"stdout":   &sinks.StdoutSink{},
 	"nats":     &sinks.NatsSink{},
+	"http":     &sinks.HttpSink{},
 }
 
 var Receivers = map[string]receivers.ReceiverFuncs{