allow for freeplay without sharing

![image](https://cloud.githubusercontent.com/assets/1767466/5175010/430ec9d6-73eb-11e4-8da8-9c26f1ccff54.png)

I don't love that the field is disableSharing instead of enableSharing, but the default case is that we want to have sharing work for freePlay unless disableSharing is set.

This also exposes the field to levelbuilder, so that we can set it to true on the one level where we currently want to.
Seems a bit unexpected for `disableSharing` to cause a `finalStage` string change. If nothing better is available to key off of, maybe track this in Pivotal or add note in builder form about this side effect for now