Check if the Security Kit module is enabled

When a user initiates a login to an IDP the module will now check to see if SecKit is installed and add the login protocol/domain to the CSRF whitelist if it is. A note has been added to README.md to inform users.

Automatically closed - issue fixed for 2 weeks with no activity.

Back-port to D7 for this could be nice for legacy sites. If I get a chance, I'll take a look.