Allow redirect to specific pages on success

So a couple of things I added for my application. 

1) A way of using the frontend social controller to pass in a uri to go to if the linking of accounts was successful (instead of just the list of linked accounts).

2) A way of using the redirect_to session data that the main login uses to redirect to that on a successful login if it is there.

3) A model function to return the active token for a specified user. I use this for sharing on facebook and twitter as the currently logged in user.