[Nimble] Add 'receiveNotification' matcher

Hmm... interesting problem. Really, users can just write the following:

```swift
expect{
    // do something
    return nil
}.to.post(UIApplicationDidBecomeActiveNotification)
```

But I guess end-users would be confused as to why the `return nil` is necessary.

I'm worried about all this overloading. But I guess the benefits for end users outweigh those concerns here. :weary: :speech_balloon:

That being said, please remove the comment and call the same `DSL.expect` method the other `expect` functions do:

```swift
func expect(closure: () -> (), file: String = __FILE__, line: Int = __LINE__) -> ActualClosure {
    return DSL.expect({ closure(); return nil }, file: file, line: line)
}
```