Update implementation of the distributed array protocol

Things done:
1. Update `protocol_validator.py` in externals
2. Update the version number exported in `LocalArray.__distarray__()`
3. Translate 'n' `dim_dict`s to the equivalent 'b' on export
4. Implement the empty dim_dict alias from the protocol

Things not yet done:
* Add support for padding
* Add support for the `one_to_one` key with unstructured (pushed to 0.3, #249)

Closes #190.