Add JSON Loader

[sktometometo]

Add JSON Loader.

This PR offers json parser implementation in pure euslisp.

{
    "a": 1,
    "b": {
        "cde": ["fgh", 1.0, true, 2]
    }
}

> (load "json.l")
> (parse-json-file "./hoge.json")
(("a" . 1) ("b" ("cde" . #("fgh" 1.0 t 2))))

In this implementation, json object (dictionary) will become an association list and json array (list) will become a vector.

So this library also offer simple accessor for association list like below.

> (setq data (parse-json-file "./hoge.json"))
> (access-alist data "a")
1
> (access-alist data "b")
(("cde" . #("fgh" 1.0 t 2)))

[mqcmd196]

Sorry for being late. Is this different from https://github.com/jsk-ros-pkg/jsk_roseus/tree/master/roseus_mongo/euslisp/json ?