Add generic method to find out if the user specifically asked for json data

By default we want to return HTML but if the user specifically requests
json this method will tell us so.
Return in json all the arguments to forward to the openid server if asked

If the user asks for json return him/her all the argument to pass to the
openid server to create the session. This will allow us to create openid
CL client without having to parse the HTML to retrieve the arguments to
give to the openid server.

Fixes https://github.com/fedora-infra/python-fedora/issues/93
Include the server_url in the json returned
Oupsy, some variables do not need to be added for each item of the loop
Make things nicer by using toPostArgs instead of toArgs

Thanks @puiterwijk
Merge pull request #94 from fedora-infra/json_openid

Json openid