use http-conduit instead of http-client in github authentication
reject github callback whose state differ from the one we previously sent (to protect against cross-site request forgery attacks)
update documentation: configuration splitted inseveral files and githublogin scope and pushing to github
added a query to check membership in an organisation, provided an option github-org is given in github section of configuration
removed github name and use login instead as it is not always present in the github response, added read:org scope depending on whether it is set in the configuration file