Rename project from Carton to Cask

_(work in progress)_

Luckily Github recently added the redirect feature when renaming repositories, so we should be fairly ok.

### TODO

- [x] Print deprecation warning if using `carton` command instead of `cask` (including test)
- [x] Print deprecation warning if using a file named `Carton` instead of `Cask` (including test)
- [ ] Rename repository from `rejeep/carton` to `rejeep/cask.el`
- [ ] Update MELPA recipe for Carton and create one for Cask (see https://github.com/rejeep/melpa/tree/cask)
- [ ] Tweet and blog about the change
- [ ] Update logo to a cask instead of carton (hopefully have a guy on it)
- [ ] Update instruction video (luxury)
- [x] Add migrate information in README
- [x] Allow the directory `elpa` if present