1.0 Release and API update

[frosty]

This PR updates Gridicons to a 1.0 release number, and updates the API:

Getting an image of a Gridicon is now a case of doing:

let icon = UIImage.gridicon(.pages)

With type inference, this can be shortened further:

icon = .gridicon(.pages)

You can still optionally specify a size:

let icon = UIImage.gridicon(.pages, size: CGSize(width: 100, height: 100))

To test

• Build and run the demo app and ensure the icons still appear
• Check that tests still pass

You can also run and verify the associated WPiOS PR (link coming shortly).