def create_user(options={})
      response = connection.post do |req|
        req.url "users", options
      end
      return_error_or_body(response)
    end
def create_user(options={})
      response = connection.post do |req|
        req.url "users", options
      end
      return_error_or_body(response)
    end
Create a user in the checkd.in system tied to the authenticating client.

 @param [Hash]  options
 @option options String :identifier            - REQUIRED, The authenticating client's internal identifier for this user.
 @option options String :email                 - REQUIRED, A valid email for the user
 @option options String :referral_token        - OPTIONAL, the referral token of the user that referred the user being created.
 @option options String :first_name            - OPTIONAL
 @option options String :last_name             - OPTIONAL
 @option options String :gender                - OPTIONAL, format of male or female
 @option options String :birth_date            - OPTIONAL, YYYY-MM-DD format
 @option options String :username              - OPTIONAL
 @option options String :mobile_number         - OPTIONAL, XXXYYYZZZZ format
 @option options String :postal_code_text      - OPTIONAL, XXXXX format
 @option options String :classification        - OPTIONAL, the internal group or classification a user belongs to
 @option options Boolean :delivery_email       - OPTIONAL, whether a user should receive email notifications
 @option options Boolean :delivery_sms         - OPTIONAL, whether a user should receive sms notifications
 @option options Integer :campaign_id          - OPTIONAL, automatically join a user to this campaign, rewarding existing known actions