Add attribute level save/update control

[ahacking]

This PR add the ability to control what attributes or relationships are sent back to the server as sometimes is it is undesirable to send everything back, e.g. read-only attributes or other meta data managed by the server or relationships which should not be managed through the model.

On your model you can now do:

var Person = Model.extend({
   createdAt: Ember.attr(Date, {update: false}),
   searchHits: Ember.attr(Number, {save: false}),
   friendships: Ember.hasMany('App.Friendship', {save: false})
});

The save: false option prevents the attribute ever being sent on a create or update, whilst the update: false option will never update the attribute, but does allow the value to be sent on a create. This control is intended to be independent/orthogonal of any adapter implementation since it controls the toJSON() serialisation of the model which occurs prior to the data being given to the adapter for persistence.