Create Endpoints with no explicit service

Add `const` method to `Endpoint` companion object for creating endpoints
with no explicit service.

Fixes #90.
I would prefer an explicit creation of the service. It's much easy to read and it follows the style that is used in Finch. We can do something like this:

```scala
implicit def futureToService[Req, Rep](future: Future[Rep]) = new Service[Req, Rep] {
  def apply(req: Req) = future
}
```