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/trig/trig.go

@@ -3,6 +3,7 @@ package trig
 import (
 	"math"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
 )
 
@@ -24,7 +25,7 @@ func (s *Trig) Description() string {
 	return "Inserts sine and cosine waves for demonstration purposes"
 }
 
-func (s *Trig) Gather(acc inputs.Accumulator) error {
+func (s *Trig) Gather(acc telegraf.Accumulator) error {
 	sinner := math.Sin((s.x*math.Pi)/5.0) * s.Amplitude
 	cosinner := math.Cos((s.x*math.Pi)/5.0) * s.Amplitude
 
@@ -41,5 +42,5 @@ func (s *Trig) Gather(acc inputs.Accumulator) error {
 }
 
 func init() {
-	inputs.Add("Trig", func() inputs.Input { return &Trig{x: 0.0} })
+	inputs.Add("Trig", func() telegraf.Input { return &Trig{x: 0.0} })
 }