Feature requests

[ypresto]

Below TODOs are found while trying to implement GCM backoff retry.

• Provide convenient way to run in background
  • Backgrounding with Handler requires their own HandlerThread. This is expensive to manage for simple background task. runInBackground() method is preferred.
  • Promise support is also preferred.
• Multiple task support
• return false to retry java if (!tryHogeFuga()) return false; if (!tryFooBar()) return false; return true;
• Callback when task won't be retried anymore
• No until() defaults to infinity (current is 0)