lib: intro the friendlier to use json::dynamic

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/json: privatly import lexer and parser

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/json: improve doc of the static module
lib/json: intro the main module json.nit

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>