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/system/system.go

@@ -8,6 +8,7 @@ import (
 	"github.com/shirou/gopsutil/host"
 	"github.com/shirou/gopsutil/load"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
 )
 
@@ -19,7 +20,7 @@ func (_ *SystemStats) Description() string {
 
 func (_ *SystemStats) SampleConfig() string { return "" }
 
-func (_ *SystemStats) Gather(acc inputs.Accumulator) error {
+func (_ *SystemStats) Gather(acc telegraf.Accumulator) error {
 	loadavg, err := load.LoadAvg()
 	if err != nil {
 		return err
@@ -68,7 +69,7 @@ func format_uptime(uptime uint64) string {
 }
 
 func init() {
-	inputs.Add("system", func() inputs.Input {
+	inputs.Add("system", func() telegraf.Input {
 		return &SystemStats{}
 	})
 }