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/registry.go

@@ -1,53 +1,8 @@
 package inputs
 
-import "time"
+import "github.com/influxdata/telegraf"
 
-type Accumulator interface {
-	// Create a point with a value, decorating it with tags
-	// NOTE: tags is expected to be owned by the caller, don't mutate
-	// it after passing to Add.
-	Add(measurement string,
-		value interface{},
-		tags map[string]string,
-		t ...time.Time)
-
-	AddFields(measurement string,
-		fields map[string]interface{},
-		tags map[string]string,
-		t ...time.Time)
-}
-
-type Input interface {
-	// SampleConfig returns the default configuration of the Input
-	SampleConfig() string
-
-	// Description returns a one-sentence description on the Input
-	Description() string
-
-	// Gather takes in an accumulator and adds the metrics that the Input
-	// gathers. This is called every "interval"
-	Gather(Accumulator) error
-}
-
-type ServiceInput interface {
-	// SampleConfig returns the default configuration of the Input
-	SampleConfig() string
-
-	// Description returns a one-sentence description on the Input
-	Description() string
-
-	// Gather takes in an accumulator and adds the metrics that the Input
-	// gathers. This is called every "interval"
-	Gather(Accumulator) error
-
-	// Start starts the ServiceInput's service, whatever that may be
-	Start() error
-
-	// Stop stops the services and closes any necessary channels and connections
-	Stop()
-}
-
-type Creator func() Input
+type Creator func() telegraf.Input
 
 var Inputs = map[string]Creator{}