Convenience methods for Response, Negotiator and Testing.Browser

Adds convenience methods to do content negotation looks something like this:

```f#
type TestModule() as this = 
    inherit NancyModule()
    do fancy this {

        get "/%s/%d/%s" (fun s i s1 (c:CancellationToken) -> fancyAsync {
            let a = "%"
            return this.Negotiate 
                    |> ResponseOrNegotiator.Negotiator
                    |> addModel (Model {name = this.s}) 
                    |> statusCode (Number 418)
        })
```
Does the same sort of thing for `Response`

```f#
ctx.Response 
|> ResponseOrNegotiator.Response
|> contents (fun s -> 
    use sw = new StreamWriter(s)
    sw.Write("banananas")) 
|> ignore
```

Also adds a `FancyBrowser` for testing. Has a better interface to construct a `Nancy.Testing.Browser`

I haven't looked at the build script. It needs to be modified so it also creates a `Fancifull.Testing` nuget package :smile: 

Have still to decide on what of this needs to be tested. Since we're only adding a small layer over the well tested stuff in nancy I don't see why we'd want to test this. Nancy also doesn't test their extension methods on response and negotiator.