[contacts] Add SeasidePerson::addAccount function

It's normally not necessary to add online accounts directly
to a contact, since they're detected automatically by contactsd,
but it can be useful.

The intent here is to allow applications to create fake (unsaved)
contacts with online account details, not to provide extensive
editing of online accounts from SeasidePerson.
Merge pull request #141 from special/master

[contacts] Add SeasidePerson::addAccount function