wrap rapidjson for lua
###json null value : rapidjson.null
##api:
##rapidjson.decode(js)
DESCRIPTION
json string to lua table,js is json string
RETURN VALUE<br/>
On success, return a lua table. On error, nil is returned
##rapidjson.encode(t)
DESCRIPTION
lua table to json string
RETURN VALUE
On success, return a json string. On error, nil is returned