Facebook SDK integration updated to use version 3.0

* When the user logs into Facebook the access token is sent to the server and saved on the shared preferences of the divice.
* When the user logs out of Facebook, the access token is deleted on the server and from the shared preferences of the device.
* Missing functionality: retrieve the stored token from the server. Currently it's only restoring the token from the shared preferences of the device, so if the user logs in from a different device, he'll have to connect to FB again. This functionality will be added on a separate commit in the future.