Added mask support to showError:, showSuccess:, and showImage:

First off - great project! I use this guy all the time. Thanks for nailing this. 

I added support to declare a mask type for:
```showError:```
```showSuccess: ```
and, ```showImage:```

I added these because in order to keep the appearance of the HUD consistent across the app, you need to add mask support for these. It just looks inconsistent / odd to have a mask for certain HUD messages, and not for others. I couldn't really think of a reason why this isn't already supported, so I just added it. 

Thanks Again!
TH