added support for message_format and notify parameters in send_direct_me...

added support for message_format and notify parameters in send_direct_message method. 

It appears that HipChat is now supporting the message_format and notify parameters in the [private_message_user operation](https://www.hipchat.com/docs/apiv2/method/private_message_user). This allows for the Will client to send the same imgs and links in 1-on-1 conversations as it can in group conversations.