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.
Nitpick: This test is fine, but it's semantics are incorrect: `example.name` is the concatenation of the example and its group's name, if it belongs to an example group. This test would break if it were placed within a `describe` closure.

I prefer the following test:

```swift
describe("World metadata") {
  it("provides the name of the current example") {
    let metadata = World.sharedWorld().currentExampleMetadata!
    expect(metadata.example.name).to(equal("World metadata, provides the name of the current example"))
  }
}
```

Thoughts? :raised_hands: