Add "image" field to JSON payload for message creation

We need to add an `image` attribute in addition to `content` and `circles`. Users can pass in a direct link to an image, and it is the client's job to render this image and provide upload services.

Example:

```
{
    "content": "A text message",
    "circles": [],
    "image": "http://i.imgur.com/...[.jpg | .png | etc.]"
}
```
