Added XML request and response support.

This adds 'xml' and 'xmlData' mirroring 'json' and 'jsonData' to scotty.
Unfortunately, this also introduces a dependency on xml-conduit.
You might ask who uses XML for REST nowadays anyway. That is a good point,
but should scotty ever want XML support, here it is.

I also added a minimalistic example on XML consumption and production.

---

This is how the example plays out:

### Valid XML body:
`$ curl -X POST -d '<hello>world</hello>' http://localhost:3000/xmlecho`

```<?xml version="1.0" encoding="UTF-8"?><hello>world</hello>```

### Invalid XML body:
`$ curl -X POST -d '<hello>world</broken>' http://localhost:3000/xmlecho`

```<h1>500 Internal Server Error</h1>xmlData - no parse: 1:13-1:22: Expected end element for: Name {nameLocalName = "hello", nameNamespace = Nothing, namePrefix = Nothing}, but received: EventEndElement (Name {nameLocalName = "broken", nameNamespace = Nothing, namePrefix = Nothing}). Data was:<hello>world</broken>```

### Server Log:
>POST /xmlecho
  Params: [("`<hello>world</hello>`","")]
  Accept: `*/*`
consuming body
  Status: 200 OK 0.000101s
POST /xmlecho
  Params: [("`<hello>world</broken>`","")]
  Accept: `*/*`
consuming body
  Status: 500 Internal Server Error 0.000155s