Rename test_state.py to test_values.py for consistency.
Add test for block resource GET result
Add test for state_{to,from}_json.
Change block cells' get() method to return the block.

This brings consistency with JS cells and slightly less need to switch
on .isBlock(). The old behavior, which is relevant for persistence, is
available as the new methods .get_state() and .set_state().
Eliminate getBlock method.