951 can model promises

This implements promise API for can.Model as mentioned in #951 . Docs are missing, but I'll add them if we agree on the implementation.

It adds the `.get` static function on the model which calls `.findOne` internally and returns a compute that holds the deferred object returned from the `.findOne` function. When this deferred is resolved or rejected compute is updated.

It has a demo similar to one in the `can/list/promise` plugin.