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/procstat/procstat.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/shirou/gopsutil/process"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
 )
 
@@ -49,7 +50,7 @@ func (_ *Procstat) Description() string {
 	return "Monitor process cpu and memory usage"
 }
 
-func (p *Procstat) Gather(acc inputs.Accumulator) error {
+func (p *Procstat) Gather(acc telegraf.Accumulator) error {
 	err := p.createProcesses()
 	if err != nil {
 		log.Printf("Error: procstat getting process, exe: [%s] pidfile: [%s] pattern: [%s] %s",
@@ -175,7 +176,7 @@ func pidsFromPattern(pattern string) ([]int32, error) {
 }
 
 func init() {
-	inputs.Add("procstat", func() inputs.Input {
+	inputs.Add("procstat", func() telegraf.Input {
 		return NewProcstat()
 	})
 }

plugins/inputs/procstat/spec_processor.go

@@ -6,14 +6,14 @@ import (
 
 	"github.com/shirou/gopsutil/process"
 
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf"
 )
 
 type SpecProcessor struct {
 	Prefix string
 	tags   map[string]string
 	fields map[string]interface{}
-	acc    inputs.Accumulator
+	acc    telegraf.Accumulator
 	proc   *process.Process
 }
 
@@ -34,7 +34,7 @@ func (p *SpecProcessor) flush() {
 
 func NewSpecProcessor(
 	prefix string,
-	acc inputs.Accumulator,
+	acc telegraf.Accumulator,
 	p *process.Process,
 ) *SpecProcessor {
 	tags := make(map[string]string)