Update to work with bevy 0.13

[j-t-d]

This isn't perfect, due to bevy's changes to TextureAtlas, but it works.

The API is now changed on finish to return the texture handle and the layout.

To use this today, update your Cargo.toml to this:

bevy_tile_atlas = { git = "https://github.com/j-t-d/bevy_tile_atlas.git", rev = "b5318c6"}