Added a proposal for post post function

to send a post
$post = $client->authenticate($auth)->posts()->postPost($server, $type, $permit, $license, $content, $file);
type is e.g. 'status', 'essay', 'photo'
TODO:
- sanitize user input, e.g. filter_sanitize_string
- attachments are not tested yet (they might need the boundary "-----------TentAttachment")