Adding setting existing token; Adding option to disable friends photos

Hey Brad,

1) Added the setToken:expiry method as you suggested. 
2) Added showFriendsPhotos property (default YES) to enable/disable "friends photos". This affects the requested permissions as well as the UI elements.

I could not test what would happen once the passed token expires.

P.S. Don't forget to tag 1.0.0, so it can be pulled from cocoapods.

Best,
Israel