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/inputs/mongodb/mongodb_data.go

@@ -5,7 +5,7 @@ import (
 	"reflect"
 	"strconv"
 
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf"
 )
 
 type MongodbData struct {
@@ -97,7 +97,7 @@ func (d *MongodbData) add(key string, val interface{}) {
 	d.Fields[key] = val
 }
 
-func (d *MongodbData) flush(acc inputs.Accumulator) {
+func (d *MongodbData) flush(acc telegraf.Accumulator) {
 	acc.AddFields(
 		"mongodb",
 		d.Fields,