Add support for "Conditional" references and reference sets

Yesterday vds asked me if Storm included a way to do "conditional references". This is a case where the foreign key relationship is one to many, but there is some flag for selecting a single item from that set (e.g. a boolean value, comparing a date field, etc). Often such a relationship would be accompanied by a database level conditional unique index.

I'd imagine that this feature could be useful for Launchpad too: an obvious example would be a person's preferred email address. The Person <-> EmailAddress relationship is one to many, but at most one of those address can be in the PREFERRED state. I'd imagine this case could look something like:

    preferred_email = Reference(id, EmailAddress.person,
        EmailAddress.status == EmailAddressStatus.PREFERRED, on_remote=True)

I realise that not all features of Reference() would make sense in t his case, but enough of them would to make it useful. Things that should work:

1. Reference.__get__(): Would need to use the store.find() code path rather than store.get(). Would need to add the extra conditionals to where clause.

2. caching of linked object: should work same as before: break relation if either object changes.

3. expression builder for "Reference == obj": Same as now, but have to make sure obj satisfies additional conditions. Checking with compile_python.get_matcher() would work for simple conditions.

Things that don't have obvious answers:

1. Reference.__set__(): setting foreign key on new object is easy. Making the object meet extra conditional expression is hard.

2. Unsetting reference: do you unset the foreign key, or do something to make the extra conditional expression fail?