fixes #1 - calling show() twice breaks the composer cancel button

- workaround for a suspected bug in MFMailComposeViewController;
  the delegate `mailComposeController:didFinishWithResult:error`
  is no longer called after pressing the composer cancel button
  and then calling `presentViewController` with the same
  MFMailComposeViewController instance

- workaround solution, per suggestion by @ckniffen is to
  dealloc/alloc/init the MFMailComposeViewController instance