Automatically flush batched writes in the HTTP sink (#31)

* Add error handling for Sink.Write

* simplify HttpSink config

* HttpSink: dynamically sized batches flushed after timer

* fix panic if sink type does not exist

sinks/httpSink.md

@@ -9,25 +9,21 @@ The `http` sink uses POST requests to a HTTP server to submit the metrics in the
   "<name>": {
     "type": "http",
     "meta_as_tags" : true,
-    "database" : "mymetrics",
-    "host": "dbhost.example.com",
-    "port": "4222",
-    "jwt" : "0x0000q231",
-    "ssl" : false,
+    "url" : "https://my-monitoring.example.com:1234/api/write",
+    "jwt" : "blabla.blabla.blabla",
     "timeout": "5s",
     "max_idle_connections" : 10,
-    "idle_connection_timeout" : "5s"
+    "idle_connection_timeout" : "5s",
+    "flush_delay": "2s",
   }
 }
 ```
 
 - `type`: makes the sink an `http` sink
 - `meta_as_tags`: print all meta information as tags in the output (optional)
-- `database`: All metrics are written to this bucket 
-- `host`: Hostname of the InfluxDB database server
-- `port`: Portnumber (as string) of the InfluxDB database server
-- `jwt`: JSON web tokens for authentification
-- `ssl`: Activate SSL encryption
+- `url`: The full URL of the endpoint
+- `jwt`: JSON web tokens for authentification (Using the *Bearer* scheme)
 - `timeout`: General timeout for the HTTP client (default '5s')
 - `max_idle_connections`: Maximally idle connections (default 10)
-- `idle_connection_timeout`: Timeout for idle connections (default '5s')
+- `idle_connection_timeout`: Timeout for idle connections (default '5s')
+- `flush_delay`: Batch all writes arriving in during this duration (default '1s', batching can be disabled by setting it to 0)