(RESEARCHING) Use OAuth2 to access Bing Ads APIs

Instead of using usernames and passwords, we eventually need to switch to OAuth2.

Unfortunately, the sandbox does not support OAuth2. I emailed Peter Becker asking when the sandbox will support OAuth2 and when username/password authentication will sunset.

https://msdn.microsoft.com/en-us/library/bing-ads-user-authentication-oauth-guide.aspx

We need to use the authorization grant flow to access the API with our aml-bing-api@admaxlocal.com account.

https://msdn.microsoft.com/en-us/library/bing-ads-user-authentication-oauth-guide.aspx#authorizationcode