Fixes a bug with not prompting for credentials
Fixes a bug with not prompting for credentials

* XmlHttpRequest objects need to have their withCredentials member set to true in order for the browser to prompt for the credentials required to authenticate with HTTP Basic, what Diigo API uses. My browser had cached them while I was developing the script, but new users' requests would just fail.
* Removed the use of const and let keywords in the unused routines.
* Updated unused readBookmark routine with API key placeholder as well.
michael-myers/ImportDeliciousToDiigo