Use an internal 'Metric' data type for telegraf metrics

As of now, this is pretty much just a wrapper around client.Point, but
this gives latitude to expand functionality more easily.

closes #564

models/accumulator.go

@@ -0,0 +1,18 @@
+package models
+
+import "time"
+
+type Accumulator interface {
+	// Create a point with a value, decorating it with tags
+	// NOTE: tags is expected to be owned by the caller, don't mutate
+	// it after passing to Add.
+	Add(measurement string,
+		value interface{},
+		tags map[string]string,
+		t ...time.Time)
+
+	AddFields(measurement string,
+		fields map[string]interface{},
+		tags map[string]string,
+		t ...time.Time)
+}