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/ps.go

@@ -3,8 +3,8 @@ package system
 import (
 	"os"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/internal"
-	"github.com/influxdata/telegraf/plugins/inputs"
 
 	"github.com/shirou/gopsutil/cpu"
 	"github.com/shirou/gopsutil/disk"
@@ -23,7 +23,7 @@ type PS interface {
 	NetConnections() ([]net.NetConnectionStat, error)
 }
 
-func add(acc inputs.Accumulator,
+func add(acc telegraf.Accumulator,
 	name string, val float64, tags map[string]string) {
 	if val >= 0 {
 		acc.Add(name, val, tags)