Fix some issues

 - add 'provide' statement for requiring this package
 - Use 'defvar' instead of setq for byte-compiling warnings
 - add autoload cookies
 - Enable docker-mode when file name is matched 'Dockerfile'.
Merge pull request #1 from syohex/fix-package

Thanks to @syohex for the following: 

- add 'provide' statement for requiring this package
- Use 'defvar' instead of setq for byte-compiling warnings
- add autoload cookies
- Enable docker-mode when file name is matched 'Dockerfile'.