I added Maven support, possibility to use custom layout, the possibility to create non-floating messages with a custom View and I also fixed a wrong behavior

I added the possibility to use custom layout for the message, since it has an TextView with the android.R.id.message. I also added the feature of using an in-layout View, instead of a floating View for the message. If this new feature is used, the custom view will be hidden, instead of removed from the layout. So, it can be reused as many times as the user wants to. I also fixed the behavior of the clear() method, since it wasn't removing the message when it was being shown.