Add paused-init feature.

[inamiy]

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 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