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/mock_Plugin.go

@@ -1,12 +1,16 @@
 package inputs
 
-import "github.com/stretchr/testify/mock"
+import (
+	"github.com/influxdata/telegraf"
+
+	"github.com/stretchr/testify/mock"
+)
 
 type MockPlugin struct {
 	mock.Mock
 }
 
-func (m *MockPlugin) Gather(_a0 Accumulator) error {
+func (m *MockPlugin) Gather(_a0 telegraf.Accumulator) error {
 	ret := m.Called(_a0)
 
 	r0 := ret.Error(0)