[Fix bug 1076865] Add alternate email addresses in ExternalAccount.

* Add TYPE_EMAIL in ExternalAccount model.
* Exclude TYPE_EMAIL from external account formset.
* Exclude TYPE_EMAIL from accounts property.
* Implement various model properties as custom attributes.
Implement UserProfile privacy aware properties in __getattribute__ method.
Merge pull request #1100 from johngian/1076865-alternate-email

[Fix bug 1076865] Add alternate email addresses in ExternalAccount.