Option to use client-side generated UUIDs

If a model has been given a `@uuid()` method, it'll be used in the constructor to generate `@cid`, and also set `@id` to the same value right away (without waiting for `@save()`).

This allows records to be created by `relation.coffee` without needing to wait for server-generated IDs to associate them. I use this for scenarios where a controller needs to build a few (unsaved) related records which it may or may not send to the server depending on user actions.

Example usage with [node-uuid](https://github.com/broofa/node-uuid):

```coffee
# Enable for all models
Spine.Model.extend uuid: -> uuid.v1()

# Enable for a single model
class Client extends Spine.Model
  @configure 'Client', 'name'
  @uuid = -> uuid.v1()
```

Here's what it can do:

```coffee
PhoneNumber.belongsTo 'client', Client
Client.hasMany 'phone_numbers', PhoneNumber

# Records can be related before they're saved
new_client = new Client
  name: 'John Doe'
phone1 = new PhoneNumber
  name: 'Home'
  number: '1-444-555-6666'
  client_id: new_client.id

# Nested relations can be created by an instance constructor
new_client = new Client
  name: 'John Doe'
  phone_numbers: [
    { name: 'Home', number: '1-444-555-6666' },
    { name: 'Cell', number: '1-443-553-4444' }
  ]
# An unsaved Client instance will be created with an ID, 
# and the Photo model will be refreshed with 2 new related records.
```

Everything works as usual if the `@uuid()` method is missing.