Added anchorPoint and initialState attrs (and update source acording to ...

...this new options)

I've added two new options that I think it could be useful:

- You could now set the anchor point in attrs. So in that way you don't have to clutter your Java code. Also you can use the method expandToAnchor().
- You could now set the initial state in which the panel will be shown (three options are posible: start collapsed, start anchored or start expanded).

Note:
I had to change several variables from final to non final because Android Studio was throwing errors about final variables not initialized!