Initial implementation of Model and Restful storage

/cc @danschultz, @sizov

This is just a first draft, it still has a lot of features missing, like:

* Error handling (not 200 statuses, "errors" key contents)
* Repeating requests several times if server doesn't respond
* Fallback to local storage
* ```ModelFactory.collection()``` needs to return some collection class, which is aware of pagination
* Missing tests for Model and ModelFactory

But this is at least something we can use to move forward.