Added missing getElementByCfi() to ReadiumSDK.Views.ScrollView

For some reasons the getElementByCfi() function is missing in ScrollView. This breaks our code when using ReadiumSDK.Views.ReaderView.getElementByCfi and the current view is a ScrollView. I'm not sure why the function is missing, but this commit does seem to do the trick. Alternatively, ReadiumSDK.Views.ReaderView.getElementByCfi should check if _currentView implements this function. Any opinions?