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:

~~~nit
# 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 login = "Morriar"
var user = new JsonObject
user["login"] = login

# Data are stored under a key.
var key = "users/{login}"

# Store the object
store.store_object(key, user)
assert store.has_key(key)

# Load the object
assert store.has_key(key)
var morriar = store.load_object(key)
assert morriar["login"] == login

# Clear all stored data
store.clear
~~~

*Copied from module documentation.*

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
En general, la doc dans le code dans la doc de l'API est a éviter.

Fait des petits morceaux de code et remonte la doc d'un niveau.