Add SSO for v3.x

[soymgomez]

Hello,

I wanted to make a PR to add the possibility of SSO in the v3.x. I understand that I should do it in the cachethq/cachet project in the 3.x branch since that is where the user model exists.

If you see this well I see to make a PR to be able to review it.

Best regards

[jbrooksuk]

This is awesome, thanks for your interest in contributing to Cachet! I'd actually build this into the core package and use the cachet.user_model configuration value to find the User model.