Support search-only PersonaStores
Submitted by Travis Reitter
Some potential PersonaStores cannot be enumerated (eg, LDAP, Facebook search, Gravatar). As such, we need to support the notion of search-only PersonaStores which only create Personas when provided a query.
This was originally a requirement for bug #646808, but I'm splitting it out to make that bug more manageable.
Version: git master