Refactoring for better readability and practice.

[chand1012]

Recently a friend of mine helped me out with a code review. Here are the suggestions that need fixed.

• The above issue.
• The variables client_id and client_secret should not be set to empty strings, should be set to None. It's faster and better practice.
• The default string "oauth.json" should be moved to a variable at the start of the file (or in __init__.py).

[chand1012]

Fixed, see here.