[WIP] Create Endpoints with no explicit service

Add `const` method to `Endpoint` companion object for creating endpoints
with no explicit service.

Fixes #90.
Do you think, it's possible to implicitly convert `Future[Rep]` into a `Service[Req, Rep]`. It's always possible to convert a value into a `Future` by `.toFuture` function, but what if we have some sort of async interface that returns futures but it's not a service it self. We, should be able to route the request to that interface. For example, there is a postgres client form `finagle-postgres` that returns `Future[Seq[A]]` on `select`. We may convert it into a service. Like this:

```scala
val e = Endpoint[HttpRequest, Seq[Int]] {
  case Get -> Root / "a" -> Seq(1, 2, 3).toFuture
  case Get -> Root / "b" -> serviceThatReturnsSeqOfInt()
  case Get -> Root / "c" -> pgClient.select("select u.id from users")
}
```

Sorry for changing my mind. But I think having a coversion from `Future` to `Service` sounds like a more general solution. Could you please try it?