Better retrying.

- Moved retry logic with retry count into `NetworkBitmapHunter`, all other hunters need not to retry.
- Now taking into consideration `NetworkInfo`. If available we check `isConnectingOrConnected()`, if not available then always assume connected and retry.
- Turning on airplane mode returns `null` for `NetworkInfo` from the `ConnectivityManager`. The `PicassoExecutorService` now properly resets thread count when this scenario occurs. Before it would maintain the old thread count which was a bug.

Tested sample app with airplane mode ON and wifi connected. Images downloaded as expected.

Fixes #169