Endpoints and method for creating new user

I added the 2 API endpoints:
* `/users/hp.json`
* `/users.json`

And also created a method `create_user_past_honeypot` that will do all the necessary steps (2 API requests, Honeypot stuff) for creating a new Discouse user.

This Pull Request is quite rough (e.g. I just put the `create_user_past_honeypot` into the Client class), but I would be willing to bring it up to shape according to your guidance.