Add testing framework for main webhook app

This commit adds a testing framework modeled after the godoc tests. It builds
webhook in a temporary directory, runs it with the supplied `hooks_test.json`
configuration, and then tests different payloads.  I use `/bin/echo` for the
test executable, so I've added build tags to exclude Windows.

Three minor (I hope) changes in functionality:

- I ended up moving everything from `init()` to `main()` because `init()` was
  firing while trying to build the tests, and it was dying since `hooks.json`
  didn't exist. I'm still not 100% sure `init()` was firing, but I didn't see
  any real need for anything to be in `init()` in the first place.

- make sure logger is using `os.Stderr`

- don't send `http.StatusBadRequest` when the Hook rules don't match.  "Bad
  Request" is used to identify malformed requests.  The request was properly
  formed and processed, so I think we should send back `http.StatusOK`.  For
  example, if I setup a webhook rule to only execute when commits are made to
  the `master` branch, we shouldn't send back `http.StatusBadRequest` when we
  ingest a payload for the `development` branch.

The test payloads are pretty verbose and could probably be shortened, but I kind
of like having an example payload for each service.  We can pare them down if we
want to do more focused, minimalist testing.

webhook.go

@@ -46,7 +46,7 @@ var (
 	hooks hook.Hooks
 )
 
-func init() {
+func main() {
 	hooks = hook.Hooks{}
 
 	flag.Parse()
@@ -87,9 +87,7 @@ func init() {
 			log.Printf("\t> %s\n", hook.ID)
 		}
 	}
-}
 
-func main() {
 	if *hotReload {
 		// set up file watcher
 		log.Printf("setting up file watcher for %s\n", *hooksFilePath)
@@ -112,7 +110,7 @@ func main() {
 	}
 
 	l := negroni.NewLogger()
-	l.Logger = log.New(os.Stdout, "[webhook] ", log.Ldate|log.Ltime)
+	l.Logger = log.New(os.Stderr, "[webhook] ", log.Ldate|log.Ltime)
 
 	negroniRecovery := &negroni.Recovery{
 		Logger:     l.Logger,
@@ -233,7 +231,6 @@ func hookHandler(w http.ResponseWriter, r *http.Request) {
 		// if none of the hooks got triggered
 		log.Printf("%s got matched (%d time(s)), but didn't get triggered because the trigger rules were not satisfied\n", matchedHooks[0].ID, len(matchedHooks))
 
-		w.WriteHeader(http.StatusBadRequest)
 		fmt.Fprintf(w, "Hook rules were not satisfied.")
 	} else {
 		w.WriteHeader(http.StatusNotFound)