Add getIdentities function

I implemented your suggestions from https://github.com/protz/GMail-Conversation-View/pull/866.

Should getDefaultIdentity() rather return only the nsIMsgIdentity-object instead of the struct? isDefault would always be true!
I think you can rewrite this in a slightly clearer manner:

```
let isDefault = (currentIdentity == MailServices.accounts.defaultIdentity);
```

then... (see below)