[libcommhistory] Report resolving status from RecentContactsModel

Clients should be able to determine if the RecentContactsModel has
unresolved events or not.
Merge pull request #39 from matthewvogt/resolving

[libcommhistory] Report resolving status from RecentContactsModel
[libcommhistory] Switch to requiredProperty instead of selectionProperty string.

So that we can provide a bit mask.