From Justin.tv API Wiki
Creates a new user. This call is restricted to whitelisted applications to prevent spam. The created user and OAuth tokens for the created user are returned.
- birthday (yyyy-mm-dd format)
- Can additionally include any user parameters that are not read-only. (see User Object)
- In order to upload a picture, set the content header 'multipart/form-data' and send a valid jpg, png or gif in the picture param.
Returns: User Object