On Windows std::optional requires C++17

fixes #13

fixes #14

fixes #7

fixes #9
fixes #10

fixes #3 
fixes #11 

fixes #1 

* 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

* Added a callback interception in login to set the access_token_ of the client on a non-erroneous login containing a non-empty access_token

* Formatted with lint

fixes #15