fixes #23

On Windows std::optional requires C++17

fixes #21

fixes #13

fixes #14

fixes #7

fixes #9
fixes #10

fixes #3 
fixes #11 

fixes #1 

fixes #4

* Added set_displayname to allow the client to set their display name

* Added call to set the user_id when login succeeds

* Added another put function for the requests that respond with bodies. Such endpoints may stop sending back responses since PUTs should not send back data in the body of the response to a successful request.

fixes #12 

fixes #6 

fixes #2