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

input.go

@@ -0,0 +1,31 @@
+package telegraf
+
+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()
+}