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/netstat.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"syscall"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
 )
 
@@ -21,7 +22,7 @@ func (_ *NetStats) SampleConfig() string {
 	return tcpstatSampleConfig
 }
 
-func (s *NetStats) Gather(acc inputs.Accumulator) error {
+func (s *NetStats) Gather(acc telegraf.Accumulator) error {
 	netconns, err := s.ps.NetConnections()
 	if err != nil {
 		return fmt.Errorf("error getting net connections info: %s", err)
@@ -64,7 +65,7 @@ func (s *NetStats) Gather(acc inputs.Accumulator) error {
 }
 
 func init() {
-	inputs.Add("netstat", func() inputs.Input {
+	inputs.Add("netstat", func() telegraf.Input {
 		return &NetStats{ps: &systemPS{}}
 	})
 }