Value collection to relationship

Hey guys, got a new one for you!

This pull request allows for MTLManagedObjectAdaptor to perform a fetch request when serializing a to-many relationship from a collection of values.

Previously, if JSON came over the wire in the following form when retrieving an object:

```json
{
  "id": 3,
  "children": [4, 7, 8, 29, 100]
}
```

I couldn't figure out a way to fetch children by their identifiers when serializing the object via `MTLManagedObjectAdaptor`.

So I built in a new method `+ (NSDictionary *)managedObjectCollectionDescriptionForKey:(NSString *)key;` which can also be used by the MTLModel property key pointing to the relationship `+ (NSDictionary *)childrenManagedObjectCollectionDescription` to define an instance where a collection of values should be used to fetch existing objects for a relationship.

I'm excited to hear what you guys think!

P.S. Will add tests as soon as I get a confirmation that this is a solid approach :)