Merge: lib/github: GitHub loader

Helper tool to download a Github repo and store it in a Mongo database.

Features:
* supports github wallets
* save current job and restore jobs after failure
* options to enable branches, commits, issues, pulls, comments and events loading

Pull-Request: #2369
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/json/serialization_write.nit
Simple merge