Hidding huds that are nor being hidden at the moment

Related: https://github.com/jdg/MBProgressHUD/pull/188

I will describe it again.
The key problem occurs when we in the same view call several HUDs that handle different processes.
But when we call the `hideHUDForView:animated:` and set `animated:YES`, we have a following conflict:
```
showing hud presync 0x9aa7980
showing hud dash 0x8dee690
hiding hud 0x8dee690
hiding hud 0x8dee690
REMOVED 0x9aa7980
REMOVED 0x8dee690
```

As you can see the HUD actually gets removed from parentView too late, which makes `hideHUDForView:animated:` work on the same HUD.

If a HUD is in the "removing" process, we don't care about it anymore, therefore it doesn't even need to counts as a present HUD for the view.