Add paused-init feature.

This pull request will enable `Task` to pause its execution on `init()`,
and manually `resume()` to start running (like `NSURLSessionTask`).

This feature will be useful in [ReactKit](https://github.com/ReactKit/ReactKit) for creating "cold signal" as well.

### Usage

```
// define task
let task = Task(paused: true) { progress, fulfill, reject, configure in
    ... // this closure will be invoked on 1st `resume()`
}

// task.state is `.Paused` at this point

task.resume()    // start running
```