Playground support

[briancroom]

Now that Cedar's test runner is usable from Swift, it would be really cool to be able to use it from within a playground as a prototyping environment. Unfortunately, this appears to be a bit blocked right now on an issue which prevents exceptions thrown in playground code from being caught properly by Cedar. I've filed a Radar and am hoping this may get resolved at some point:

https://openradar.appspot.com/radar?id=4971823104000000