Added Delete support for /users endpoint

[jooooel]

Deleting users requires a DELETE to the /users endpoint using parameters in the URL. The body is ignored.

I removed the non functioning delete method and added two new. One using the Intercom id, and one using the custom user_id.

Hope you like it :)

[mgoodfellow]

LGTM :+1:

I will prep a new nuget package in the next few days