eliminating '_uri' for correcting inconsistent naming on the event_subscriptions resource.
reflected @ConnorDoyle 's review comments in (https://github.com/mesosphere/marathon/pull/159#issuecomment-35949406).

- change JSON's attribute names to `camelCase`
- change the identifiers using underscores in Scala to `camelCase`
- change api endpoint urls to `camelCase` (`/v2/eventSubscriptions?callbackUrl={url}`)
- _NOTE: I left underscores in protobuf messages._