Fixed URL for OAuth2 login, and added token disable support

This change fixes the OAuth2 overload of `BuildAuthorizeUrl` to use www.dropbox.com as the base URL instead of api.dropbox.com, per the API docs. It did not seem to work using api.dropbox.com, although I'm very new at this and might have been doing something wrong. At any rate the change makes the code match the Dropbox API docs. I also added support for the `disable_access_token` API function.

This is my first ever GitHub pull request so please let me know if I'm doing it wrong or missing out on etiquette. I believe I matched the style of the existing code, at the very least.