Add nbimg, a utility for creating splash screens for Android and winCE

Note that I have omitted a `test` block only because running `nbimg` without any arguments prints the usage message, but then returns an error code.  I'm not sure of a useful test then.  If this is okay, (especially as this is a really simple utility) then I'll just leave it, but if there's an easy way to write a useful test, I can include it.