add parent record to default value function

[knownasilya]

Would probably need some docs too. What's the use-case for this?

[knownasilya]

ping

[knownasilya]

@VincentMolinie I can merge once feedback is addressed

[dwickern]

This was implemented by #417. The fragment attributes' defaultValue has the same signature as ember-data's attr. defaultValue: (record, options, key) => ...