Add xml response type

This introduces a XML as a new response type. I adapted an XML escaper from here: https://gist.github.com/atesgoral/156752 as I don't think `sanitizer` will escape XML. I'm not that familiar with node so I'm not sure if where I put the `escapeXML` method is the correct place (`./lib/helpers.coffee`).

I wrote this mainly to play around with node and see if I could add a new feature, I'm not convinced XML support is a long sought after feature! :)

The XML response will be as follows:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<foaas:response xmlns:foaas="http://foaas.com/fuckoff">
  <foaas:message>Fuck off, Bob</foaas:message>
  <foaas:subtitle>- Alice</foaas:subtitle>
</foaas:response> 
```

I took a stab at a namespace, I'm sure someone can think of something more appropriate!