Added support for Google AppEngine

[koAlech]

Added a chainable function AppEngineHTTPClient to override the http.client used to post the errors.
On a project running on GAE, you should pass a *http.Client returned from urlfetch.

raygun, err := raygun4go.New("appName", "apiKey")
if err != nil {
log.Println("Unable to create Raygun client:", err.Error())
}
client := urlfetch.Client(c)
raygun.AppEngineHTTPClient(client)

[miko-code]

+1