Relationship conditions

Could be useful to add conditions to a relationship:

``` js
relationships: {
  recentAlbums: {
    type: 'hasMany',
    local: 'id',
    foreign: { table: 'album', key: 'bandId' },
    conditions: {
      released: {
        operation: '>=',
        value: 2012
      }
    }
  }
```

Thoughts?
