Fixing Camera.prototype.capture function, which is returning HTTP 404

Dropcam recently removed the `/api/cameras.get_image` method and it is now returning HTTP 404. We can fall back to the Nexus API and the `get_image` method to retrieve screenshots for the `Camera.prototype.capture` function to get around this.

More details here: https://github.com/rsgalloway/dropcam/issues/7