Allow non-ID foreign keys

[brummett]

Allow object accessors that link to a non-ID property for the remote object, and do the join in SQL if the data is in the DB. For example, return all the OtherObjects where their "their_name" property matches my "name" property.

Some possible syntaxes:

other_objects => {
    is => 'OtherObject',
    id_by => { name => \'their_name' },
}

or

other_objects => {
    is => 'OtherObject',
    where => { their_name => \'name' },
},

Before actually getting into the implementation, the latter is preferable. The accessor/mutator should follow the other indirect object mutators by automatically assigning the linking property if passed an OtherObject instance.