Control chars should be escaped in JSON

As you can see in this image, the most current version of ServiceStack.Text isn't escaping control characters like the spec requires (and your competitors do):

![servicestack text control char encoding bug](https://f.cloud.github.com/assets/85714/1120856/0a14a608-1ab1-11e3-9081-c2ffdef97b7a.PNG)

This pull request should fix the issue - I didn't do any performance testing, though.