Add attribute to customize animable view inside ExpandablePanelView

With the current implementation library users has to use as animable view the child at position 0 or 1 depending on the expandablepanel:invertBehavior attribute. I've added a new styleable attribute to customize the animable view using a animableViewId. 

With this new feature you can add more views to your ExpandablePanelView and put the animable view at 0 or 1 position is not mandatory. This new attribute is not mandatory and library users doesn't have use it if they don't have more than two views inside ExpandablePanelView or the new child is above the second child view.

I've updated two fragments with this new attribute and added more information in the README.md to explain the usage.