Added Auth class to manage user authorization process.

1. Added Auth class.
2. Added `auth` property to ColorMe.
3. Added unit tests.
4. Updated README.md about setup and authorization process.
Added Auth class to manage user authorization process.

1. Added Auth class.
2. Added `auth` property to ColorMe.
3. Added unit tests.
4. Updated README.md about setup and authorization process.
Changed ColorMe's auth property's visibility.
Merge branch 'feature/auth' into develop