added basic http redirects for #54

Added a simple factory for creating url redirects.
It can be used with Finagle Paths or raw strings.
You can also optionally set the string content and the Respond type.

Some examples:
```scala
  object SomewhereService extends Service[HttpRequest, HttpResponse] {
    def apply(req: HttpRequest) = Redirect(Root / "somewhere").toFuture
  }
```
```scala
  object GoogleService extends Service[HttpRequest, HttpResponse] {
    def apply(req: HttpRequest) = Redirect("http:www.google.com").toFuture
  }
```
I think it should be enough having just two methods: 

```scala
def apply(url: String)
def apply(path: Path)
```

, which  always generate `SeeOther` respond.