Flush notices on panic fixes #27

[sypher7]

Problem

Documentation suggests capturing unhandled panics as such:

func main() {
  defer honeybadger.Monitor()
  // ... Exploding code here ...
}

However, the Monitor() function will often re-raise the panic's error and exit the application before it can be sent by the backend, since that happens asynchronously.

Solution

Force notices to Flush() before re-raising.

[joshuap]

@sypher7 thanks, this looks good to me!