Merge: lib/json: introduces json store a simple json data storage system.

Store and load json data.

This simple system can be used to store and retrieve json data.

## Usage

Create a new JsonStore or reuse an existing one.
~~~
var store = new JsonStore("store_dir")
~~~

JsonStore can store json of type JsonObject and JsonArray.
~~~
var color = "red"
var obj = new JsonObject
obj["color"] = name
obj["code"] = "FF0000"
~~~

Data are stored under a key.
~~~
var key = "colors/{name}"
~~~

Store the object.
~~~
store.store_object(key, obj)
assert store.has_key(key)
~~~

Load the object.
~~~
assert store.has_key(key)
var red = store.load_object(key)
assert res["name"] == name
~~~

Clear all stored data.
~~~
store.clear
~~~

*Copied from module documentation.*

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #1019
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>