Kn rapidjson

First attempt incorporating rapidjson into colony.

Major parts:
* Parse: parses stringified json into lua tables and deals with parsing errors
* Stringify: stringifies lua tables into strings

Stringify replacer functions/arrays are working as well as stringify spacers.
Review all of this. Recommend changes to the Lua/C as much as your heart desires (optimizations/fixes welcome)
`to_default` doesn't exist on `rapidjson` afaik? Either way, if you replace the "default" handling with

```js
rapidjson.object_start(handler)
rapidjson.object_end(handler)
```