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)
"unsupported types" can actually just be a final `else` clause on this if-else chain. You don't need to screen for userdata/function/thread types, just let it all be handled indiscriminately.