Use transform to store attribute values in deserialized format.

This forces the attribute values in `_attributes` to be stored in the deserialized format.

This essentially makes the internal values much more consistent. Currently if you bind a non-string value in a template (`boolean`, `date`, or `number`) the internal attribute is represented in string form (since `{{input}}` will always be a string). This is an issue if you use `Em.computed`'s on the attribute that expect a given attribute format.

For instance:

Storing a `count` field (`DS.attr('number')`), and using an `Em.computed.sum` accros multiple records starts simply appending strings after one is set with an `{{input}}`.

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There may be a wonderful reason that this is not done at the moment, but it wasn't obvious to me.