Allow users to link together different social accounts

[CAFxX]

Using the account-* packages, it appears there's no easy way to give the user the ability to link additional accounts. A couple of scenarios where this could be useful:
1 - User signs up with mail/pwd, but then he wants to use also his facebook account
2 - User signs up with facebook, but then he also wants to link his g+ account (because e.g. some of his friends might be on g+ but not on facebook)

[glasser]

Duplicate of #1133. (It's a pretty complex feature to implement. We're aware of it.)