Add support for background gradient

[apassant]

This PR adds support to use a background gradient (using CAGradientLayer) to the empty state view.

README has been updated to instruct how to handle it, with the example of a simple B&W gradient.

[dzenbot]

This is interesting, although it needs a way to remove the gradient if nil is passed.

[apassant]

Good point, I'm happy to make the change and update the PR if you think that's worth merging with this change.