Make httpbin.HTTPBin implement http.Handler interface (#100)

This is a very small improvement to developer ergonomics.

Author: mccutchen

README.md

@@ -62,7 +62,7 @@ import (
 
 func TestSlowResponse(t *testing.T) {
 	app := httpbin.New()
-	testServer := httptest.NewServer(app.Handler())
+	testServer := httptest.NewServer(app)
 	defer testServer.Close()
 
 	client := http.Client{

example_test.go

@@ -12,7 +12,7 @@ import (
 
 func TestSlowResponse(t *testing.T) {
 	app := httpbin.New()
-	testServer := httptest.NewServer(app.Handler())
+	testServer := httptest.NewServer(app)
 	defer testServer.Close()
 
 	client := http.Client{

examples/custom-instrumentation/main.go

@@ -13,12 +13,12 @@ import (
 func main() {
 	statsdClient, _ := statsd.New("")
 
-	h := httpbin.New(
+	app := httpbin.New(
 		httpbin.WithObserver(datadogObserver(statsdClient)),
 	)
 
 	listenAddr := "0.0.0.0:8080"
-	http.ListenAndServe(listenAddr, h.Handler())
+	http.ListenAndServe(listenAddr, app)
 }
 
 func datadogObserver(client statsd.ClientInterface) httpbin.Observer {