Implicit converter from Json to HttpResponse

This add support for automatic conversion from `Service[Req, T]` to `Service[Req, HttpResponse]` when an `EncodeJson[T]` is provided, such as:

```scala
trait Json

implicit val encode = new EncodeJson[Json] { def apply(j: Json) = "42" }

val service: Service[HttpRequest, Json] =  ???

val e = Endpoint[HttpRequest, HttpResponse] {
  case Method.Get -> Root / "a" => service // should be implicitly converted into http
}
```

When type information is not provided to compiler, you can still force the conversion using the `.asJson` method:
```scala
val e = Endpoint {
  case Method.Get -> Root / "a" => service.asJson
}
```

Futures are also supported:
```scala

val e = Endpoint[HttpRequest, HttpResponse] {
  case Method.Get -> Root / "a" => (new Json).toFuture
}
```
Likewise, we can also force conversion with the `.asJson`

Addresses #104 
Don't you think it should be `toHttp` since it's already a JSON.