Shared example metadata

I had opened this PR on the [wrong repo](https://github.com/orta/Quick/pull/1). 
Doh. Adds the ability to grab metadata about the current Examples from the sharedWorld.
`NSExcetion+Callsite.swift` was removed in 6df89119a626b1f21771d89dfef4781cf887051a; remove it from your `.xcodeproj`. :point_up: