Make binding HTTPS servers even simpler via SSL-Config

**Issue by [ktoso](https://github.com/ktoso)**
_Friday Apr 22, 2016 at 07:54 GMT_
_Originally opened as https://github.com/akka/akka/issues/20381_

---

Something like this for inspiration:

``` scala
implicit val system = ActorSystem()
implicit val mat = ActorMaterializer()
implicit val dispatcher = system.dispatcher

val sslConfig = AkkaSSLConfig().mapSettings { s =>
// TODO implement this: s.withServerKey
s
}

val https: HttpsConnectionContext = Http().createServerHttpsContext(sslConfig)

val routes: Route =
get {
complete("Hello world!")
}

// bind to default port for HTTPS (433):
Http().bindAndHandle(routes, "127.0.0.1", connectionContext = https)
```

Start with the Scala entry points named in the issue: AkkaSSLConfig, createServerHttpsContext, and bindAndHandle. Review how the proposed SSL-Config flow should supply server settings, then verify that the example can bind an HTTPS server through the connection context shown.

Written by the indexing model from the issue text.