Create public models for telegraf metrics, accumlator, plugins

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

plugins/outputs/cloudwatch/cloudwatch.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/outputs"
+	"github.com/influxdata/telegraf"
 )
 
 type CloudWatch struct {