Add content when posting to Groups

Adds the ability, when posting to a Group, to include content.  Content can contain a title, an image preview URL (submitted-image-url), and a description (these are all optional) and must contain a link/URL (submitted-url).  

Details of the corresponding LinkedIn API can be found at https://developer.linkedin.com/documents/groups-api#create

Implementation detail: the GroupPost and GroupPost.Content classes do not extend the LinkedInObject since it automatically adds the extraData field to the JSON payload in the request and LinkedIn rejects these.