Created LaunchImage asset set

@andymoe 
* well this was embarrassingly simple (if non-obvious)
* appears 4" iPhone letter-boxing is removed when a Default-568h@2x.png is available
* previous auto-layout setup has translated across to the 4" screen without modification
* consequently, only change was to create a launch image asset set
    - chose this instead of setting specific resources, to save future headache, as it seems that Xcode kept using the similar images in the kxmovie submodule bundle instead of the selection I was making (apparently known bug)