Added a new method in TeleportClient to send a message with a custom ResultCallback

[gabrielemariotti]

This feature allows to use a custom ResultCallback sending a message in the TeleportClient. The previous behaviour is preserved. Also provided a little javadoc for changed methods.

An example to use the new feature (useful to update the doc):

ResultCallback<MessageApi.SendMessageResult> callback = new        
       ResultCallback<MessageApi.SendMessageResult>() {
                @Override
                public void onResult(MessageApi.SendMessageResult sendMessageResult) { 
                             // Provide your custom code
                }
            };
mTeleportClient.sendMessage(HELLO_WORLD_WEAR_PATH,null,callback);