Rework the Airbrake hook

Rework the Airbrake hook to:

a) change the interface so that the Airbrake credentials are stored in
an unexported struct, `airbrakeHook`, which is instantiated using the
`NewHook()` method

b) send log entries where no 'error' field is set to Airbrake, using the
`entry.Message` string as the message sent to Airbrake but continue to
allow the passing of error types using the 'error' field

Update the tests accordingly, assuring that the correct message is
received by the Airbrake server.

Also update the examples in the README, which would not have worked with
the previous implementation of the Airbrake hook.

examples/hook/hook.go

@@ -3,21 +3,16 @@ package main
 import (
 	"github.com/Sirupsen/logrus"
 	"github.com/Sirupsen/logrus/hooks/airbrake"
-	"github.com/tobi/airbrake-go"
 )
 
 var log = logrus.New()
 
 func init() {
 	log.Formatter = new(logrus.TextFormatter) // default
-	log.Hooks.Add(new(logrus_airbrake.AirbrakeHook))
+	log.Hooks.Add(airbrake.NewHook("https://example.com", "xyz", "development"))
 }
 
 func main() {
-	airbrake.Endpoint = "https://exceptions.whatever.com/notifier_api/v2/notices.xml"
-	airbrake.ApiKey = "whatever"
-	airbrake.Environment = "production"
-
 	log.WithFields(logrus.Fields{
 		"animal": "walrus",
 		"size":   10,