fixed broken setActivate / state_activated on API 11+

As I mentioned in issue #52 the newly implemented choice mode support breaks the use of setActivate / state_activated on API 11+. 
To fix this I have moved the Checkable-implementation into a subclass of WrapperView and added a check in `getView` of `StickyListHeadersAdapterWrapper`.

I also fixed a strange bug I encountered when using this library with EndlessAdapter (cwac-endless). (see lines 50-55 in `WrapperView`)