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)
It looks like this duplicates a lot of functionality that is in `json_stringify`. Can you deduplicate this by just calling `json_recurse` from the body of `json_stringify`?