Draw divider for list header/footer views

Dividers for header/footer view is drawn in `ListView` by default. I made a patch to imitate this drawing using pattern already being used in the library; making use of `WrapperView`. Header/footer view is now wrapped by `WrapperView` and managed with a little more care.

*WrapperViewList.java:112-121*
As each divider in `WrapperView` is drawn by the view below, the last divider at the bottom has to be drawn differently. (fyi: divider at the bottom is only shown when items of the `ListView` doesn't fill all the `ListView`'s space. Refer to the screenshots in [this SO question](http://stackoverflow.com/questions/14199274/separator-divider-after-last-item-of-listview))

Further improvement would be to implement `android:header(footer)dividersenabled`, giving on/off functionality.

If you would merge, please review the code throughly. Though I tested several cases, I may have not caught all the edge cases. I'd be glad to take any feedback.