Add streamed update support.

For my app I need to be able to push updates as JSON over web sockets and update models as needed. I also need to have FilteredRecordArrays update as expected so that new records are always picked up. This PR bundles those changes together for me to use ember-model in this way with my app. This PR supersedes #210.

Hopefully I've covered off enough of the edge cases for this to be useful to others as well.